The heart starts functioning before birth and it is difficult to deliver the blood to the head and the other upper organs as they are located above heart level. Kneeling during prayers renders its work much easier.
And as per Salem Ben Abi Al Gaad, said: A man said: Musaar Arah mn Khazaa: I wish I prayed, so I was relieved. They seemed to mock him. So he said: “I heard the Prophet (PBUH) saying: “O Bilal Call for prayer, give us rest with it.” Sunan Abi Dawod (4985) corrected by Al Albani.