My guess is it has nothing to do with the mechanics in use.I'd be surprised if the players are being turned away because you don't allow UA. My guess would be it is because you don't allow feats and multiclassing (which we would have even if the PHB were the only 5e book).
I did my share of looking for a group through ads and the like back in 3e. Through accessdenied.com mostly.
The key factor - pretty much the only factor - was "Will I enjoy playing with these people?" I'd have played Pathfinder if I liked the people and that's what they played.
(I'm glad my group doesn't want to play Pathfinder)