Because one of them is actively promoting and favouring viewpoints many people find abhorrent.
The fact it’s owned by a billionaire isn’t the major concern for most people.
Yes. Having centralized ownership (to whatever extent) is a concern for sure, but it’s a hypothetical concern in and of itself: “what if the leadership does bad things?” Is different from “the leadership is currently doing bad things.”
Decentralization helps. But if the networks effects aren’t behind it, jumping from platform to platform when things DO get bad is also viable.
Honestly, being willing to jump to a new platform is possibly the most effective way to stop platforms from fucking you over too hard.