I think the main reason is becuase the lack of gravity makes it much harder to crouch

This is likely true. Recall that Apollo 11 landed soft and failed to compress the landings struts. Armstrong practiced jumping up to the lowest rung -- about three feet off the footpad. When it came Aldrin's turn, he had to make two attempts before he could do it.

You have to physically (i.e., using muscles) flex your knees in lunar gravity. On earth you can just relax your muscles and let gravity do most of the flexing. Plus the accordion joint at the knee was designed only to allow comfortable flexion to a certain degree, that required for walking.

Its like trying to walk underwater.

Yes and no. Part of the difficulty in walking underwater is moving against the much denser fluid medium. But in terms of the lack of traction due to buoyancy it's comparable. The astronauts were packing 360 pounds of their own mass plus their equipment. While the weight is reduced on the moon, the inertia is not. But weight in turn determines traction, which means they had twice the mass and 1/6 the traction, leading to a dozen times the hassle.

The reason most commonly given by the astronauts for why they didn't perform gymnastics (except for Jack Schmitt, who did perform gymnastics) is that the center of gravity was so far to the rear that they feared falling over backwards while jumping. Armstrong said he nearly did this and it scared him, so he didn't do it anymore.

The other astronauts who witnessed John Young's "mere" 19-inch jump weren't disappointed. They were actually surprised that after only a few minutes in lunar gravity he was already so agile.