There are many complicated mechanics in the Pokemon game that deal with battle. Hidden Power, IVs, EVs. They are all complicated and hidden systems of the game that general players do not even bother with because of the difficulty and effort. Its used for the more experienced battle veterans.
Pokemon is a kids' game, but that doesn't mean that complicated systems are not in the games. Majority of children that play don't learn about EV training until a few years after.
It would be that complicated if introduced and would cause a large amount of trouble. Though its already been shown that there isn't room for second type buttons to appear on an attack's information page. Only thing they can do now is coded effects like Hidden Power, Natural Gift, Nature Power and Secret Power. Which would mean that they would not be Dual Typed attacks.