I thought this wouldn't be as bad, but when I checked it does indeed seem the case. Looking at the most active posters of last month (December), I counted 41, and checked which Wikipedia should be called their home (I have not checked non-Wikipedia Wikimedia wikis, which at least three (Brian McNeil, Gerard Meijssen and Bryan Tong Minh) are more active on), based on frequency of recent edits.
Of those 41: * 24 had the English wikipedia as their main wikipedia * 7 had another wikipedia as their main language (Daniel Arnold on de:, Lars Aronsson sv:, Luiz Augusto pt:, Milos Rancic sr:, Waerth th:, Yoni Weiden he:, effeietsanders nl:) * 3 were about equally active on en: and some other language (Michael Bimmler de:, Florence Devouard fr:, Jussi-Ville Heiskanen fi:) * 6 I could not identify with a Wikipedia username; 4 of those would probably be en: users judging from their name, the other 2 were unclear * 1 does not really have a home Wikipedia, having the majority of his edits on another project (Gerard Meijssen, counted as nl:)
All in all, English users outnumbered other-language users by 28 to 11 (counting the 3 doubles as other-language and the 6 unknowns as 4 English, 2 not counted). That's slightly over 70% English users, whereas they're less than half of our volunteer population (as far as I know).
As for the reasons, the fact that this list is written in English is an important factor. I myself read and write English quite well, but there will be plenty of contributors who do speak some English, but for whom reading in English is much more tiring than doing so in their own language, and writing English would be even worse.
Having said that, I agree with other writers in this thread (like Notafish) that the high amount of often low-quality or off-topic replies is not a good thing, although I doubt whether it's more of a factor for non-English speaking people. I have been thinking whether it would be possible to get a lower-volume version of this list, but no way of making a selection that would even be a little effective while at the same time not leading to even more heated discussion seems doable.