The reason is simply a technical one, if DLC was enabled if only one player can use it, all the art and xml assets would have to be transmitted to all the players that do not have it, so that their games could let you see the DLC civs units etc. Given the weak state that the MP code is in right now, it could not hand that.
So their solution was only to enable DLC if everyone has it, and therefore get around the need to transmit the assets.
So we are stuck like this until they fix MP, this is the same reason they do not enable MP mods and custom maps, the games network code just can't handle that right now.
CS