An opaque bottle makes it difficult to confirm that the UV light is running if the light's built into a cap.

I can't speak for everyone, but I find current products like the SteriPen more useful because they're container-independent. A bottle with a built-in sterilizer means that I have to carry that bottle, even if I'd rather bring a different size or material. And if the bottle doesn't share threads with a common design (e.g. Nalgene 1L, Sigg...), destroying the bottle potentially leaves the expensive cap useless if, for example, I'm traveling in a country where your company has no distributors.

A cap for Nalgene bottles with a built-in sterilizer might save enough space over a separate sterilizer to be worth the downside of not being able to use it in a glass or someone else's bottle.