It's mostly his style of tennis. Murray like to hit some junkballs that lack both power and heavy topspin. On hardcourt he gets away with it, because those balls bounce lower and it's harder to be aggressive on them.
On clay Murray is easier to attack. It's his own fault really. If he ditched his crappy junk shots and went for more heavy forehands or at least actual moonballs (high bounce is also hard to attack, especially from the BH side) but instead he often gives the opponent command of the points.
That might work well vs the lower ranked players even on clay but later in the tournament, QF-> when he faces players in form or the top10, then it's not very efficient. He needs to win matches on his own rather than hoping for tons of errors or that hurricane winds saves the day. He is clearly capable of hitting both heavy forehands and backhands so his problem is mostly in his head. His junkshots are simple not as efficient on clay, yet he plays clay like any other surface.
He has started hitting his fh much heavier and more aggressive post Wimbledon but its still a forgone conclusion that he will not amount to much this clay court season as that loss the stan indicates.