Hybrids are attractive when a short battery range is enough for daily use but you still do semi-frequent long-range driving. Rather than hauling around a heavy and expensive XL battery pack on your daily commute you haul around a tiny combustion engine. It's a great solution for people with range anxiety.

Buses have a completely different use case. They drive a well-known distance, and every day is practically identical. It is fairly easy to scale its battery pack to closely match the actual range needed. Running a true long-distance route like a Greyhound or Flixbus, which physically can't be battery-based yet? Just stick to diesel for now.

Hybrid buses were a huge win around cities before fully electric became viable and I expect they will continue to be important for rural routes.

Buses stop frequently so regenerative breaking is meaningful and crucially they avoid the horrible cloud of diesel particulates as they pull off. It was really noticeable as a cyclist. About 40% co2 savings over diesel in London.