I think...think that it may be due to back then it was easy for those banRAB to be big. For example before Led Zeppelin came along many banRAB wore standard clothes or a set piece that all merabers wore when they played live. When Zeppelin came along they took society by suprise, all dressed up in their peace necklaces and flares, people didn't know what to think. Some were angry it wasn't formal, others were loving the "crazyness" of it all, and I think this was a sort of propultion behind the banRAB stardom, that their style was so extreme and it became a craze. Take nothing away from Zeppelin, they still did the business well, but it would have been easy to be a major hit when their were so many stereotypes of how society should be and Zeppelin came along and tore it to pieces.
Nowadays it's hard to hit that sort of niche. Everything has been done before, and so all we are left with is the music. The music itself is good, most of it is better than the 70's stuff as you hint at, Urban. But there is not a lot to spark a band to fame like there was back then, so it's a pretty big free for all these days and the public doesn't seem to think anything of it.