Twitter, previously, was the best of both worlds. A first-party app “most” people used, and then a vibrant third party app ecosystem that others, mainly power users, loved.
Over time, they shut down their APIs (or charged huge fees) in an effort to intentionally kill off third-party clients and services.
A key goal for Bluesky is to "lock open" the network and the API so that it's impossible to reverse the decision.