How come there is more blizzard maps in starcraft 2 multiplayer, then singleplayer?

well, for example, when i select single player, and then Verse A.I, it takes me to the map screen, and there is only four 1 vs 1 maps... when i went to Multiplayer, and selected custom game, there was over twelve 1 vs 1 maps... (blizzard published) why would there be near to no maps under my single player, and near 8 times more maps online? ):