Uhhh, maybe because it has a higher capacity battery than the Thunderbolt??
I put a Rezound battery in my Thunderbolt yesterday morning ($20 at the VZW store) and it lasted all day without charging. First time that's ever happened.
IOW, all I'm saying is that the Rezound MIGHT not be any better than the TB. It might just be the fact that the stock Rezound battery is better than the stock TB battery. 1620mAh vs 1400mAh, I think.
I was all set to spend the big bucks to ditch my TB and get a Rezound or a Nexus, just because of having to charge my TB multiple times per day. Now that I've experiencing the Rezound battery goodness, I may end up keeping my TB. As far as I know, the Rezound doesn't actually DO anything that my TB doesn't (unless you count Beats, which I don't give a rip about). The screen on the Rezound is actually slightly narrower than the TB screen and the phone overall doesn't feel any smaller.