Because in this way, the shoes of the person praying on his left side will be on the right side of the one praying.

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When any one of you prays, he should not place his shoes on his right side or on his left side, unless there is no worshipper on his left side (then he may place them there), and he should place them between his legs." In the narration of Janab al-Dawraqi, these words are mentioned: "And he should not place his shoes on his left side unless there is no one on that side," and he did not mention the right side.