Because of the power issue? I'm having a hard time buying that. Apple has so many power sucking features on the watch already that require the user to use restraint that I can't see a white face contributing substantially to that. And even if it did, a simple disclaimer when the user selected a white background face is all that would be required, so the user knows they are shortening their better life with that choice.
And why no custom watch faces? Why no announcements from third parties about them? It's first and foremost a timepiece. It does the one thing with its appearance that a traditional watch can't do and that's change its face design. So is Apple really that concerned about power, or is it their concern for the design style? I mean it's more for the user than anybody else.