The same reason Vipers are being replaced by Reapers. They're cheaper to buy, operate, and maintain. The B-1 is becoming more and more expensive to operate and maintain as it ages. (Much like the Block 25 Vipers were) Something the B-52s have seemed to avoid?
...and completely useless against a conventional enemy. There's a reason those flying snowmobiles didn't see combat in Libya until a month after OUP began. As for the B-1R, I'd love to see these operating in conjunction with the F-22A in its 'mini-AWACS' role. You then have the ability to take down a numerically superior enemy (North Korea, PLAAF/PLANAF) with only a few dozen of your own aircraft.