Earlier quoted context omitted.
Maybe for the same reasons the German Wikipedia is so large. I'm not sure what they are, but I suspect: It's a large language sphere, speakers enjoy relative wealth and the luxury of time, there is a cultural tradition of cultivating and archiving information. All of them may make it more probable someone would submit translations. In addition, here specifically, the online community was well-developed in the late 90…
I love that you synthesized that comment on the fly then actually took the time write and share it. In the back of my mind I have a maybe-never project brewing that would entail finding Wikipedia pages that only exist in one language. English would of course have many unique pages. I wonder how many non-English page translations would be rejected on notability grounds, just due to sheer cultural differences.
This makes sense, because if one wiki has, say, 60% of the content (and depth) of some other wiki, it will break it down into pages differently.
Being able to speak both languages, I often find it instructive to browse the pages on a subject on both English and German Wikipedia, as the structure and depth of the material will often vary either way. It's sometimes unfortunate, but other times also possibly warranted that these wikis cover subjects to a different degree.