The competition is poor: Linux still has quite a lot to go in terms of distributing commercial software. Mac users are also probably more ready than Linux users to spend money on software.
I wouldn’t know how that compares when looking only at computers where the sole or main user can personally make purchase decisions, though, and that’s the market many small startups target, at least initially.
I guess Windows does worse there, but I wouldn’t dare guess by how much.
Also, it's probably easier to develop for a *nix platform since it's more open than DOS.
Sources: Just speculating, I can't back anything up 100%