Because the people who own the game have to write it? I suspect that all the really successful games will eventually end up on all mobile devices, but in the mean time you get what you get. Developers won't write for a platform if they don't think there is enough money in it.
Try searching in the App Store for the names of the developers you like. Zynga has a few besides FarmVille, if you are willing to use the iPhone versions.