That style of cockpit was all the rage there for a few years. I would imagine the people sitting down stair on the B52 would have like to have kept the original version.
I think it was a manufacturing question.
Fabricating a nice bubble canopy is a lot harder than making a canopy out of several smaller pieces. Getting acrylic plastic to lay down in a mold with
compound curves and have a finished product with good, distortion-free optical quality- it's complicated. There's a narrow temperature range for the plastic neatly taking the desired shape. A bit too warm and it will slowly flowing like really thick liquid (look closely at a 100 year old glass pane in old house, the bottom part looks "runny" and the same sort of effect can happen in an oven for manufacturing aircraft windows). A bit too cool and it won't flow smoothly, it will bunch up. A lot too warm and acrylic starts offgassing and smoking.
All that but in a thick canopy for a pressurized airplane is another manufacturing challenge.
If the windshield area is to be strong enough to survive bird strikes and enemy fire up to a given standard, that makes it more complicated.