Apple doesn't want you making Watch apps that only tell the time
The Apple Watch may come with gobs of customizable watch faces out of the box, but that doesn't mean that you can install any face you like. Apple has updated its App Store developer guidelines to expressly forbid Watch apps that are primarily designed to tell the time -- it can be a feature, but shouldn't be the focus. You'll have to make do with the existing choices for now.
However, there are signs that more watch faces are coming. Apple's help guide suggests that firmware updates could include additional built-in faces. Also, it's important to note that Apple hasn't launched its promised native app kit yet. It's too soon to say whether or not Apple will open the doors to third-party faces at some point down the road. It took a few months after launch for Android Wear to officially support additional faces, so it's not surprising that this kind of functionality is missing from Cupertino's wearable on day one.
