I’m not rocket surgeon, but I have to imagine redundancy is part of it.
A B52 has eight engines. Statistically, I supposed that a loss of an engine or two is greater there than on a two or four engine plane. The result of an engine loss on a B52 would be the so-called “dreaded seven engine approach.”
Sidebar — apparently the B52 is finally getting a new engine, cleaner and easier to maintain. I’ve heard the current engines described as “pigs” by someone who had been in the service and had maintained them.