Plants vs Zombies 2 starts when you go back in time to find the taco, but why don't you find it?

Because you are in the wrong timeline, if they just went minutes ago, Crazy Dave can eat it again, and again, and again.

Another theory may be because Dr. Zomboss stole the taco, so in your endless taco search, you fight zombies.

Everything is plan of Dr. Zomboss to make you lose your brains.

But this is all your fault, because Crazy Dave doesn't needs you, if you watched PvZ 2 trailer, you wll see that he saved the city from zombies, that he's a hero. Think about it.