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.
There also is an OS called Windows that has more users than those two combined.
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.
I wonder it's because there's so many Windows users, so few Linux users, and that middle-but-still-reall-small number of Mac users that it seems like a good market to enter into. On Windows there's likely an overwhelming amount of new products for the platform, on Linux there's not enough people willing to try your product for it to be worth the dev time, and on Mac it's still small, but who knows plenty of apps are Mac-only and they seem to be doing fine.
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%
Windows needs way more testing to launch on. Many more relevant OS versions. Widely divergent hardware vs a handful of supported models that all have pretty nice specs.
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.