It's possible the stone itself simply had special properties. After all, the sequence is "meditate at the altar, then move the stone". Granted, it's not specifically mentioned in the dialogue that only a master can move that particular stone. Then again, it's possible the altar could have some sort of adverse effect? The idea perhaps, that the meditation there actually clouds the mind, with only the most serene, or the most potent wills, able to resist its effects?
The latter seems particularly likely if it's set up as a test that someone would fail no matter what.
As this guy says, it's the meditation that's the trick, only a master could meditate properly in such a dangerous area. The boulder itself is not particulary special, I think, it's just a matter of strength in the force, but the meditation first, is tricky because as you relax the mind, you become more aware of the danger around you (the droids attacking everybody), noises stop being in the background, and your own doubts become more evident. All preventing meditation.
It has been suggested that the person you are helping isn't yet a padawan, this is wrong, he is a padawan, and this is part of his trials. To suggest that a trainee who has yet to master enough to become a padawan would be subjected to such a dangerous locale is slightly silly.