pistol pete was tough enough on clay too: he won the Italian open.
RG is just not that easy to win.
certainly the RG+Wimbledon double is not easy.
it took 30 years for somebody to come along and do it after borg.
And now you will have 3/4 people doing and possibly a lot more in the future. Surface being all the same it helps.
Sampras did beat some 'decent' players like Muster, Kafelnkov, Courier, Agassi, Bruguera, Corretja (twice in Rome Masters) on clay. Playing two weeks of tennis on clay was just too much from him to ask especially when Wimbledon was just around the corner.