I think it has a lot to do with a couple of things, which are tone and maybe the development of steel guitars in terms of complexity
A lot of early steel guitar has quite a bright raw, tone, which is fantastic and you dont hear so much these days.
This may have been to do with the simpler construction of the guitar and the changer in particular (see threads about all pull vs push pull changers)..
Since those days efforts have been made to make guitars support more complicated changes and to make them more playable and it is possible that this has changed their sound.
Also in teh early days of pedal steel, the players would have been lap steel players initially so may have had a different approach and a technique developed more towards lap steel playing. This may also have shaped the sounds they went for.
Nowadays, and since then, I think a lot of people get into pedal steel without having done a lot of lap steel before, so what they learn is how to do things with pedals where earlier players might have done things with their bar hand.
With teh more complicated changers came more options, so the modern PSG includes new sounds.
Also effects, amps, production, changes in taste, the evolution of music in general.
But in general modern steel sounds are much fuller and less toppy, maybe with more effects, more emphasis on pedals
Vintage: (simple changer, bright sound, nothing simple about what he plays!)
Modern(ish): Loaded pedal steel with a much softer tone. No flies on him either!
Of course this is just a generalisation and there are loads of players out there all doing their own thing and have been since the dawn of PSG!
Just my 2c
