Probably to simplify things, and to make things a bit easier... by making them harder.
If you had, e.g., Combusken & Blaziken on your team when you fought an Aggron, wouldn't that make the level super easy, because both Combusken & Blaziken are part fire and part fighting, meaning their attacks will do x4 damage?
The game requires you to either a) grind or b) pay, so by having dual types changes the dynamic of the game, and you can cruise past on x4 type effectiveness without having to do either.
I'm also guessing the creators of the game wanted the players to use as many Pokemon as possible (that's part of the fun) and if there were dual types you'll probably just end using the same batch of strong Pokemon and never levelling up any of the others that you could also use.