I haven't the slightest idea why anyone would voluntarily convert to ANY religion. It is my view that all religions only perpetuate superstition, of one form or another. Most people are born into a religious group, the same group their parents where born into. This was the case with myself, but I stepped away from it.
People will join church groups, for social reasons primarily I think, and therefore enter into the religion promulgated by that church. Many of these groups perform good and honerable services to their community. But the propaganda which accompanies those services is pervasive.
And of course, there's always the "convert or die" method of recruiting new members. That's always filled the churches in the past. "God told them to" may not be valid, but "that soldier with the nasty sword told me to" will convert thousands!