We don't know, neither Twitter nor Musk have said a word about it (that I've noticed, anyway).

They never mentioned publicly, they never communicated it.

But you can likely guess that this is about ads.

I don't think they actually shut down the API, they just revoked the API keys for popular third party apps. If you're using the API to build something other than a replacement for the official app, then I believe they still allow you to use it.

If they did shut down the whole API I was ready to make an argument about how public APIs can be costly to maintain because they're not used by first party software. But again, that's not the case, so I think we're left with the ads explanation.

I’d guess Ads and it’s confusing the enhancements and rapid product development they are shipping.

Makes sense.

Elon didn't know there were third party clients and immediately shut them off when he found out.

They also seem determined to show people the algorithmic timeline, so a client that actually listens to you would fight that.