Technology

Jet engines are better when you want high altitude flight and high speed. Turbopropeller engines (like that of the Tu-95) are far more fuel efficient at low altitudes, if you don't care about speed. Thus, with turboprops you gain far more strategic range than with jets.

Big bomber planes don't generally fly at very high altitudes, and high speed can mess with their targeting. So turboprops are more suited for the job.

Also, turboprops are lighter than jet engines (giving more weight allowance for payload or cargo).

Furthermore, turboprops are far safer around explosions. If a jet engine catches fire it can easily explode, due to the volatility of jet fuel and the constant air intake, unless you shut it down quickly enough. On the other hand, if a turboprop catches fire, it just sputters out and stops working. You can understand why this would be an advantage for a giant slow-moving bomber aircraft.

Turns out turboprops still have their place. It's not meant to go really fast or to be super maneuverable. It's purpose is to fly up, deploy a lot of missiles, and then go back home.