Why does 3 phase motor have more torque than a single phase motor?

A simple way to look at it is to assume you were spinning a wheel by hand and you were spinning it harder once per revolution. If you applied a load of some sort to the wheel it would slow over a period of time depending on the load. If you continued to manually try and keep the wheel spinning at the same rate, one revolution at a time this would effectively be adding torque to the wheel once per revolution. If you manually mimiced a 3-phase motor you would be manually spinning the wheel 3 times each revolution. This would basically apply three times the torque if each spin used the same force as the single spin did.