I've had lots of problems finding decent BT mice. One thing I like in a mouse is a button that I can assign as a 'back' button for web browsing. Most BT mice are lacking in features (extra buttons and whatnot).
The only possible reason I've been able to think of for BT mouse models being less popular is that perhaps additional licensing fees need to be paid by the manufacturer. Maybe there is enough demand that manufacturer's can't ignore the BT models, but not enough demand that they will make BT versions of the fancy wireless models.
If I have built-in BT on my laptop I sure as heck don't want to dedicate a USB port to a stupid wireless dongle, no matter how small it is!
-Pete