The surrealist movement grew out of Dada, which was a reaction to WW1 and the introduction of machine guns and mustard gas. "If this is what the pinnacle of science and reason has gotten us, we must undo it"
Clock faces are always flat surfaces- wall clocks, wrist watches, sun dials, etc. The gears and hands require rigid, flat surfaces.
If you're going to put a normal item in a surrealist painting, and choose a clock, making it a melting clock is essentially an obvious choice.
Fair enough. But are we certain that renaissance watermelon were more like current watermelon than this picture portrays? How do we know? (I don't know the answer, but if someone wants to continue the thread, I'd be interested to read more.)