For me too this was a big question, but the answer is in their incompetence. They deserve a Darwin award on eliminating themselves. They could've tweaked their API indeed, to accept ad through 3rd party. Even they could come up with a business model that both 3rd party and them would earn money. All these would also need time. The time that the 3rd party was asking to even adopt with their current "model" of API, they even didn't give "that" a chance.
Lemmy and kbin and others, for sure have the potential to eat the whole reddit. Reddit was nice for its simplicity, and it is definitely not hard to reproduce. The more "algorithm" reddit introduced, the worse it became.