Well it certainly wasn't music. Hence their popularity in the USA.
But Mr Clark was one of the sharpest guys in the business. He produced all their records and leased them to record companies and retained all the rights. I also think he paid his band members a salary and kept all the royalties for himself. He later bought the rights to Ready Steady Go since when he has been able to lease the footage to every nostalgia TV show - of which there have been many. And his musical Time (with Cliff Richard!) made more money
Most pop/rock stars were dim, just bothered with "the music" and were robbed.
Mr Clark didn't care about music but just the money and he is as rich as Croesus.