This is mainly due to the fact that there is no "stable" Wikipedia -- things change so quickly that citing Wikipedia makes it very difficult for anyone to actually look up whatever you were citing. If there were "snapshots" that were widely distributed, say at the end of each year, one could simply cite those snapshots.
There is actually, an effort to change some of this. The Encyclopedia of Life (eol.org), which imports Wikipedia articles about species, has a process underway where curators will be able to edit a Wikipedia article, mark it as "curated", and then have that expertly reviewed article available for reference. Think of it sort of as an EOL branded version of the article, lending more weight to its accuracy. The main article can still continue to be edited and changed.
Fast, cheap, good: pick two.