Because there are a multitude of clients that work with it. it’s open. It’s not a walled garden. You aren’t stuck in yet another horrid browser app.
Also for some purposes the lack of history can be an advantage. For a channel that’s real-time social interaction, people coming and going and only having access to the things that happened when they were there can be a positive.
I don’t even think that’s trolling. If done well with listening to and responding compassionately to people, it might even be productive.