There are a few problems with Apple Watch apps.

1. The watch requires an entirely new form of design, and until someone hits it right, it's hard to encourage companies to put forth the required effort to build a great watch app. As this great piece on Medium points out, the Apple Watch is like designing for augmented reality whereas a phone is similar to virtual reality.
2. Using apps on Apple Watch requires two hands (one for your wrist holding the watch, and one to touch the watch screen). Most people can use their phone with one hand, and it carries more functionality - so oftentimes using Apple Watch apps just doesn't make sense.
3. The Internet of Things is still undeveloped. The watch as a remote for all things connected to the internet is a powerful use case, but lagging IOT development leaves this potential basically untouched.
4. Siri is still in the works. It works pretty well now, but it hasn't been opened to developers yet and its understanding is still not completely developed. Siri enables one hand use of the watch, which is important in building a useful watch app. Once Apple supplies a Siri API the watch will become a much more valuable device.