I really wonder why. Maybe they think it can't do enough (whatever "enough" means), maybe they can't figure out how to monetize it, maybe they don't like using the watch themselves, who knows?
I've only spoken to one developer (that I know of), and he didn't care for the watch at all. He was wearing it, but said that he thought it was slow and didn't do enough things. The app he's most responsible for also showed neglect, and is still, IMO, far behind competing apps in the same genre.
In this case, it's not WatchOS's fault, nor can I blame the hardware -- it's the developer's responsibility.