There weren’t. You just like British bands of the 60s and 70s better, and think that translates into them actually being better, because you mistakenly believe it’s an objective rather than subjective matter.
But it’s not. It’s wholly subjective.
Personally, most of my favourite artists/bands of that era were, in fact, American — Bob Dylan, Lynyrd Skynyrd, The Allman Brothers Band, Creedence Clearwater Revival, The Band (American/Canadian), Crosby, Stills, and Nash (token Brit in there), Aerosmith, ZZ Top, The Ramones, The Doors, James Taylor, etc.
So I challenge the false premise of your question. Next time, before you ask “why” ask “if”.