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.
I wouldn't blame developers. Blame Apple.
The watch, when it first came out, was hugely limited. Apps had to run from a phone and there was no access to many of the components of the watch. This killed a lot of momentum which may've been gained due to the launch of the watch.
Then watchOS 2 comes out with native apps, but this requires a large revamp if you've already made a non-native app. You basically have to write a whole new mini app.
Now, combine this with a total lack of any useful app discovery on Apple's end (no top charts, no way to filter watch-compatible apps in the App Store, no initiatives like free app of the week) and no way to charge extra for an Apple Watch app and you can see why developers may be ambivalent towards the platform. Why waste hours and hours creating a watch app that targets a small subset of iPhone users, when you could concentrate on updating and monetising your iPhone app or creating new iPhone apps which will reach tens, perhaps hundreds, of millions more people?
It may be viable for large developers, but for independent ones it just doesn't make financial sense until the Apple Watch becomes so popular it would be detrimental to their business not to have one.
I love my watch, and I enjoy creating apps as a small hobby on the side. But I'm not going to plough resources into creating watch apps unless Apple give watch developers a bit more support.