re:
I don't know that Apple has ever shared an opinion or reason one way or the other on this. There are a number of possibilities that I can think of:
- They are holding out doing licensing deals with other watch makers as they have for Hermes, to keep watch faces as a way to make some money for both companies.
- They are worried about performance or battery issues for customized watch faces
I believe it's a possibility that they will do licensing deals but am not sure how likely it will be. Licensing costs money and I don't know they'd think it worth the effort for 'regular' Watch faces.
Performance and battery issues would definitely be a consideration for the Watch. Apple almost always uses battery life as a selling point.
My money says Apple doesn't want just any yay-hoo submitting a Watch face that they will have to examine for trademark infringement in addition to performance or malicious code. Related to that is crappy looking design. I had a pebble for six months before the Watch shipped. Notifications were great, but the UI was horrible and even worse were 99.9999% of the watch faces that users and 'designers' made for it. and about 99.999% of that 99.9999% infringed on trademarks. But nobody seemed to care, maybe because it was Pebble. If Apple did that, there'd be a Take-a-Number machine outside of a certain Texas court. Apple has already had a problem with a clock face in OS X.
Apple cares a lot about design and none of those Pebble faces would fly. Cheap, tacky watch faces would cheapen the brand. At one time you'd see dozens of flashlight and f**t apps in the App Store. Now, not so much to not at all. I like that. So while I want to see new, elegant, and cool Watch faces, I'm glad they're not (or at least not YET) open to outsiders.