Apple has not disclosed why they decided this, and we are not allowed to discuss Apple's decisions or policies here.
With that said, I would wager its because, unless the iPad is a Cellular model, its pretty pointless. And even if it is a Cellular model you can only really use navigation Apps. You won't be able to make or receive cellular calls with an iPad, or send and receive SMS messages unless there's a Paired iPhone nearby, and at that point, it makes more sense to just use the iPhone with Carplay rather than the iPad.
There's really no advantage to using an iPad with CarPlay. It's easier to just use the onboard navigation and media center in that case with whatever phone the users of these iPads may have.