Basically, a lot of the vintage amps could double up, because cabs were bigger (modern bass cabs have a smaller trend and are thus more power hungry), but more importantly, output transformers were beefier. The Hiwatts and Sound Citys used beefy Partridge transformers, because men were men and could be expected to carry them around. Modern guitar amps tend to scrimp on the transformers and power supplys because guitars don't need the bottom end, plus guitar cabs haven't got smaller, 4x12s are still the same as they were in the 60s.
Also, modern tones tend to be cleaner and more bottom heavy, or at least more bottom heavy, that needs lots of power. If you want a guitary tone, guitar amps will be fine.