It is almost a fact now. Xenforo rules the forum industry. Almost everyone wants to use Xenforo, but if they do not, it is possibly due to factors that are not Xenforo's fault. I am personally no fanboy. But Xenforo made some smart moves in the last decade.
I am not saying that other software is not doing the right things. But look around. Almost all new commercial-like projects are using Xenforo. A lot of websites I know have chosen Xenforo over their previous software.
Yes i have seen many look for second hand xenforo license and start websites using Xenforo. But i am not a fan of the software.