There's no practical limit to the number of people who can create user accounts and "contribute" to Wikipedia, so technically it can never become *full* of communists or any other group of like-minded ideological adherents - the number of communists it can take in and support is effectively unlimited.
As for why Wikipedia attracts communists, that's easy enough - communists tend to take a dim view of the concept of intellectual property, particularly as regards controlling the physical means of its production. Internet publishing costs are practically negligible compared with traditional publishing, and in most countries there's almost no effective legal accountability for internet publishers who at least claim to have their "users" do all the work (whereas the legal penalties for traditional publishers who print libelous or privacy-violating material can be crippling). While some effort is being made to address the problem of free/easy copyright violation on Wikipedia, and obvious copyright violations are nearly always removed when discovered, the site is nevertheless founded on the notion that most intellectual property should be freely expropriated, synthesized, and given away with no recompense to any original copyright holders that might still exist.
Communists also like the idea of a centralized propaganda distribution point, and a relatively small inner circle (some would say "cabal") of people capable of censoring anything that criticizes or competes with the controlling ideology. That way it's easier to gain popular support without having to resort to inconvenient things like the democratic process.
It should be noted that fascism takes a similar approach, which is why you find lots of them on Wikipedia too. However, in many cases, people mistake extreme libertarianism for communism and/or fascism, due to a variety of similarities in overall outlook. For some people the distinction is almost meaningless, but if you're not one of those people, then extreme libertarians actually outnumber both communists and fascists on Wikipedia.