According to its English Wikipedia article, Wikimapia has licensing issues due to tracing from Google's satellite imagery without the correct permissions. Many WMF projects have policies against linking to content which infringes on copyright. (Note: I have a conflict of interest as an OpenStreetMap editor, but Kartographer relies on OSM data in any case.)

Wikimapia data also tends to be months or years out of date, even in countries with more active contributors like Russia, and is generally less developed than OSM. This is only from my personal observation, though, since there isn't an easy way to compare the entirety of both databases.