The explanations there are clear and concise, but simpler than Wikipedia.
Wolfram doesn't have the OR problem (original research).
I've added intermediate-level "translation" text to a few Wikipedia articles, and every time I do this I know I'm at risk of being reverted for OR.
QED for the Layman is a masterpiece of original explanation—and forbidden territory for Wikipedia contributors.
Second, it's very hard to avoid saying something false when interpolating between the basic and the advanced material.
When I've tried this myself, I've estimated that I was hitting around a 90% truthfulness, with the other 10% ranging from vaguely correct to outright howlers (and me not being able to discern the difference).
I consider myself a fairly severe fussbudget in matters of accuracy, which means I trust my estimate that I'm falling short. Except for the experts who wrote the expert material—some of whom are no good at any other level—I'd rate myself fairly high. And I still don't think my intermediate contributions are quite up to encyclopedic standards (and so I mostly only dive in when the article starts out in a pretty bad place).
Unlike the simple level, the intermediate level is precise enough to get yourself into real trouble, here and there, if you're not a subject expert.
The editors who contributed the advanced material, so far as I've noticed, tended to be the 2005-2007 heyday crowd making highly substantive main edits, and not necessarily sticking around for editorial maintenance, or even to assist a less expert author trying to step in and fill the expository gaps.
First and foremost, Wikipedia is process driven, not outcome driven. People need to bear that in mind, and be happy it's as good as it is.
My least favourite articles are the mathematics-heavy articles where 90% of the text is derivational, to the degree where the main points are encoded in lemmas. What I've noticed on these pages is that it's very hard to dive in in any kind of small way. You almost have to first break the existing page's back to steer the page in a different direction.
The final class of pages I've noticed are pages that were basically abandoned 75% finished in the first place. These can often be improved with a quick effort. But if you try to add too much text, you'll fail to provide enough cites (that requires real research). In my experience, one cite attached to a few added sentences usually survives.
And then if you get reverted, the page goes back to the same state, with no warning for the next fool who comes along and tries to make the same edit.
That's what I hate most. Many editors revert a contribution aimed at fixing a problem where they view the fix as problematic, with little concern that the original state was also problematic, while taking no ownership whatsoever of the pre-existing problem.
Now I don't care if 10% of my edits get reverted (be bold), but above that level it begins to feel like a giant waste of time, so I'm careful not to be so bold as to ruin my will to participate in the first place. (One sees many bitter former editors show up in these threads who didn't figure this out soon enough.)