OK, sorry, couldn't resist writing this long thing, which has been brewing in my head for years...
So... the Jews' "affiliation" with education, whatever "affiliation" even means, may be an explanation, but it's only a hypothesis, and it's very generalized. It needs proper proof with sociological methods and comparison to people who speak other languages. If anybody runs such research, I'd be curious to read it, but until anybody does, it's just a hypothesis, and I don't want to use it as an actual explanation.
There are a few other cultural and organizational things I can think of, which probably influence the Hebrew Wikipedia's activity. They are relatively more solid than general "affiliation with education", but all of them are nevertheless based on my intuition and not on research and calculation, so be super-careful when using them for anything.
The first thing will sound obvious and transparent, but it is actually substantial: a lot of Israelis expect to search the web in Hebrew and to find a useful result at least in some cases. I don't have a number, but if I had to guess, I'd say it's well over 70% of the population (and I mean all Israelis, including speakers of Arabic, Russian, French, and other languages). It may be similar for most European languages, but I don't have statistics. I mean, it's certainly around 100% for speakers of English, French, German, and Spanish, but I don't know the numbers for Greek, Albanian, or Latvian, for example.
Again, I'll emphasize that I don't know the number for Hebrew either, but it's probably high-ish. And I am sure that these numbers are very low for pretty much all languages of Africa (except Arabic and maybe Swahili, Afrikaans, and Somali), and for most languages of Asia, except Chinese, Japanese, Korean, Indonesian, Thai, and maybe Hindi, Thai, Burmese, and Malayalam. This, in part, happens thanks to the relatively high amount of useful content in the Hebrew Wikipedia, but it's not only that; there were successful Hebrew-language websites before 2003, when the Wikipedia began. Examples are Ynet, Globes, Tapuz, and others. And Hebrew was used online even before the world wide web, using BBS and other technologies. So the Hebrew Wikipedia didn't appear in an online vacuum. And if we go even earlier in history, modern elementary schooling in Hebrew has existed since the 1880s, and education pushes people to expect to use Hebrew for most things, rather than some foreign language. (I'm not mentioning traditional Jewish religious schooling in Hebrew. It has existed for millennia, and it certainly helped with establishing modern secular schooling, but by itself, it has rather different purposes.)
Why did people in a small country, with a relatively high percent of people who can read English (around 30% report that they now English well, and 30% report knowing "some English"), and with a technically challenging alphabet (right to left...) bother to use computers in their language? I don't exactly know. But this brings me to the second point:
Tech education and industry are relatively well-developed. The military has used computers since at least 1959, and quite a lot of Israelis learn advanced computer usage there, and later use these skills in their careers. Also, the military required the use of Hebrew in its equipment, so technology for that had to be developed early. Commercial electronics and software companies, which developed original technology, have existed at least since the early 1960s (Chromagen and Elbit are famous examples, and there are many others). Math, chemistry, engineering, and physics higher education has existed since 1912, and a lot of people insisted on learning it in Hebrew and not in German, English, or other languages; see War of the Languages (although, to be honest, there's a certain shift to English in higher education in recent years). Now, since at least some understanding of technical and educational things, such as markup languages and citation rules, are important for Wikipedia editing, this certainly helped. If anyone can measure these things systematically and compare these metrics to the situation in other countries and languages, it would be very interesting.
One last thing I should mention is that precisely because the country is so small, it is relatively easy to organize physical-space meetups of Wikipedians. I'll give the Russian Wikipedia as an extreme opposite example: its editing community is huge, but the attendance at its annual conference is very small, with fewer than 50 people. I find it really strange: these meetups take place mostly in Moscow, which has more people than all of Israel, or in Saint Petersburg, which is almost as large, but because they are perceived as meetups of all Russian Wikipedians, people from other cities rarely come there, as it's far away and expensive. In Israel, there are usually two meetups a year, they are perceived as countrywide, and coming there is easy by bus, train, or car. They are usually attended by about 100 people or more. Intuitively, successful physical-space meetups probably contribute something to community development and activity, although again, I'm not entirely sure. Most European countries are between Israel and Russia by their size, and I'm curious how does this work there, and how does this correlate with the activity.
(Comment: I keep comparing Israel to European countries because they are probably the most relevant comparison by population size and also by culture. For example, it's well documented that Hebrew language activists in the late 19th century drew inspiration from language revitalization movements in Eastern Europe of the same period, for example from Czech, Bulgarian, and Ukrainian. America and Australia have just four major languages, all of which originate from Europe. India, China, Korea, Japan, Indonesia, Thailand, and Burma have active Wikipedian communities, but they are probably different culturally, and I'm less familiar with them; there may be some relevant things to compare, but it's difficult for me to pinpoint them. African Wikipedians mostly edit in English, French, and Arabic, and with the exception of Afrikaans and maybe Swahili, the communities in Africa's own languages are only beginning to emerge.)
In the very end, I need to respond to your comment about "subtraction":
- You don't need to subtract the newcomers who learn Hebrew as a second language. They do read and write the Hebrew Wikipedia. In a way I am one myself; I started editing the Hebrew Wikipedia fourteen years after I started learning Hebrew, so maybe I was not exactly a "newcomer", but it's definitely not my first language. And there are some editors who started editing after learning Hebrew for much less than fourteen years. Most importantly, people who live even for a short time in Israel need to learn to at least read Hebrew, and reading Wikipedia is the most important first step to editing Wikipedia.
- You don't need to subtract Haredim either. Some Haredim indeed avoid the secular Internet, but a lot of Haredim do use the web, and they do read and edit the usual Wikipedia. Some use voluntary censorship tools, like "Rimon", and this also creates occasional technical issues (e.g., they unintentionally break existing content while editing), and others just use regular reading and editing. Many of them also have administrator rights, come to meetups, etc. So no, don't subtract them. Certainly not all of them.
I hope this helps. And I have to emphasize yet again: practically everything I wrote here is based on intuition, so use it super-carefully, and feel free to mistrust it completely. And if you know sociologists, sociolinguists, and historians, give them these topics as research ideas ;)