I think there are a number of reasons for this, bass was almost seen but not heard in the past, but now even us oldies expect to hear more bass. Just try learning the bass part of a 60s song on an old car radio. To some degree, we have gone power mad. In the 90s, 300 watts was considered a monster. However, you only have to read how many people are using the micro amps, Elf, Gnome etc to gig with. These output about 100 watts into 8 ohms and will keep up with two 30 watt guitar combos and an acoustic drummer. The difference between 90 watts and 100 watts cannot be heard.
Of course, some might respond that valve watts were more powerful than solid state, nonsense. Valve amps compress nicely but can only output around 40% more power at maximum distortion, the same as a solid state amp of the same power.
So why are all my three amps 700–800 watts? Strangely, I use one of them as a spare that can be used either for bass or PA. The main reason is that they are there, and I can afford them. If we think back to the good old days when guitarists had 100 watt valve stacks, 300 watts were not enough (tell that to your ears) and 300 watts either Valve or Trace was the minimum you needed. However, playing with a couple of Fender Twins, AC30s etc in a pub/club, 300 watts are more than enough. As my cabinet is 8 ohms, 350 watts is comfortable with headroom (remember, solid state amps are usually quoted at 4 ohms. At 8 ohms, they will usually output 50-60% of the 4 ohm power).