The 300m users. Basically most social media platforms work off a 30/70 split with 30% of the users providing content and 70% absorbing it. Twitter is more 10/90. If you were late to the game you're basically screaming into the void and will never get followers so you abandon the platform.
The monazitation. This is twofold. One is that the 10% that produce content use it to monetise themselves, but not twitter. So Kim Kardashian will get paid $300k to advertise a product but little, if not nothing, of that will go to Twitter. Also, if you are advertising on Social Media, Facebook has 1.3 billion users and is much more focused towards groups. So not only do you get more reach, you get a much more targeted reach.
The troll problem. (This one isn't backed up by data but more my opinion.) Reddit is a social media platform where everyone is anonymous. Facebook is a platform where no-one is anonymous. Twitter is in the dangerous place in the middle. So if Gabourey Sidibe appears on the red carpet in a Versace dress and Versace tweets out "Looking good Gabourney!" people can respond with "Go back to Africa you fat, black n**ger bitch!" with very little repercussions. It doesn't take much to see why brands would abandon it