I think the answer to this is mainly in my ALSO: edit above.
The size of the file could be a big problem on many systems. Fat32 is a pretty common native filesystem for mobile readers so they can be accessed through windows, linux and mac easily and the maximum filesize limit would probably be exceeded by this ePub (4GB).
You also lose a lot of the 'magic' of Wikipedia by not being able to jump around easily via the internal hyperlinks (at least on most eReader devices today). I could never use Wikipedia to my liking without some kind of tabbed browsing functionality. My wikipedia jump tree usually has MANY branches (as in, I start with reading one article and end up with 27 open firefox tabs only loosly related to my original article
)
On another note, lets not forget the very dynamic nature of the project itself. Sure, there are many articles that are in great shape already, but there is always work going on in adding/overhauling much of the content. So there is the issue of continuously having to update the ePub, and I don't think there is any easy way to do this. We'd either have to periodically re-download the entire ePub to keep it current, or someone would have to devise a tool to grab updated data and inject it into the current ePub. Even with the periodic injections, these updates would likely still be pretty large given the scope and number of daily changes to Wikipedia.
eReading in its current form is pretty friendly towards linear reading, maybe some simple content searching and jumping around with a TOC, but definitely not friendly towards dynamic content such as Wikipedia.