Because this action abrogates the action and command of the Noble Prophet (peace and blessings be upon him) which is contrary to this action.

It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) performed prostration, he would place his knees on the ground before his hands. In the narration of Ahmad and Raja, these words are mentioned: I saw the Prophet (peace and blessings be upon him) that when he performed prostration, he placed his knees before his hands.