It has to do with the fact that cost and card strength are not linear. Chapel is an extremely strong card, to the point where it's hard to price it. If you cost it high enough, being the first player to get a Chapel may well decide the game. But, Chapel on its own is not a strategy. So if instead you cost Chapel very low, it's accessible to everyone, and gaining Chapel doesn't become a crucial achievement. It's a bit like Alms - $0 for a card costing $4 is crazy cheap, but as it's available to anyone, that's not an issue.
Huh, i guess this means a basic cantrip trasher could in fact cost $2, with the same reasoning as Chapel. I'd argue it shouldn't have abilities beyond trashing then, that make their own strategy - which in turn means it becomes probably too similar to Chapel itself.
Edit: Royal Carriage does not need to wait if you want to use it immediately. Unlike Throne Room, it never misses and multiple can be used on the same action.