Adverse selection. If your differenciating feature is "you can say what you want without getting banned", your userbase is going to tend towards people who get banned on other platforms. In todays climate, that is mostly the far right.
Compounding this is PR and network effects. Once they become known as the place for the far right, then more far righters go there, and more distant people avoid it.
Compounding this is PR and network effects. Once they become known as the place for the far right, then more far righters go there, and more distant people avoid it.