Because of inertia. There are entire industries that were built around Twitter. For years, it was an incredible networking opportunity that you were missing out on if you weren’t active. For example, many artists used twitter for discoverability; They could post their art on Twitter, and it would get much broader reach than on other social networks.

This is why substitutes like Mastodon have struggled to take off, and it’s why even the early adopters still crosspost everything to both twitter and Mastodon. Mastodon simply doesn’t have the user base required to have that same kind of discoverability. It would need to reach a critical mass level where it’s able to sustain itself without twitter. And it’s unfortunately not there.

Whether it will ever reach that point is up for debate; The same way Reddit’s scummy practices were a huge boon for lemmy, only time will tell if the same will happen to twitter. The issue is that the vast majority of users simply don’t care about a negative experience on the site. Sure, there are vocal critics, but those are often the minority who are extremely incensed and will be the most likely to change. But once those critics have fled, the vast majority still remains on twitter and now there aren’t any critics pushing for change.