I have been in a number of fixed wing syndicates over the years, and without exception they have all worked out well - even one that had 20 or so members. Availability has never been a problem, as nobody ever flies as much as they think they are going to at the outset. 20 hours or so per year max is about par for the course in my experience.
At the moment, I am in a group of 3 owning a PFA aircraft. It lives about 10 mins from all of us and rarely gets flown more than once a week. Obviouisly one major attraction of a helicopter is keeping it in your back yard and maybe prospective users are deterred from group ownership by the perceived inability to do this freely. However, I think the fundamental problem is that rotary pilots are just much fewer than f/w and therefore the syndicates that do exist are correspondingly fewer. To get a PPL(H) these days sets you back £10k, which reduces the number of new entrants considerably. Like most things, it is a numbers game and there is no simple way to change this.