I think cost of ownership and operation would be a factor here. The B-52 airframes aren’t getting any younger, and could probably do with some high bypass turbofans (they seem to talk about getting four on there frequently).
I guess by using a C-17, you get a modern aircraft with modern cost of ownership, using fuel at a far more reasonable rate. And for the times when you aren’t dropping loitering munitions out the back, you have a valuable airlift asset that you’ll utilise a hell of a lot more than a B-52.
Yes, the performance is less, but more than adequate to fly over MANPADS type threats. And for the more hardcore air defences, well, are you really expecting to fly a B-52 over those any more successfully? Somehow I think neither will be flying overhead until the SEAD types have done their bit.
The B-52 is nice for putting massive tonnage of bombs onto target, but this isn’t a trick needed. All you need is a carrier to haul these things out there, and hey presto you have a whole DAY’S worth of precision bombardment.
Same applies for using a B-1: Why do you need a swing-wing afterburning heavy bomber to drop these things? The C-17 might not be cheap to buy, but operating is a huge portion of the life-cycle cost of any platform, and the C-17 can see a lot more return on the initial investment (e.g. performing the air-lift mission even in peacetime), for lower per-hour cost. Oh, and using C-17 as the platform to carry these loitering munitions is probably preferable as a means of support for keeping open the C-17 line.