> Which developer wants to miss out on serving potentially millions of users?
You are forgetting that most developers already have millions of potential users on the iPhone with a market at least 30x bigger than the Watch's, and iOS has none of the limitations the Watch has.
Right now a Watch app requires an iPhone app, is hard to monetize, it cannot be sold on its own, it's slow and frustrate your users, etc. The only upsides are for Apple. It might sell really well because the core functionality provides enough value, but developers need to see a clear return for their efforts and right now is not there.