Why? A. Ermac is more powerful then Kenshi, and B. Ermac actually help trains Kenshi and his powers to their full potential as payback for Kenshi releasing kenshi from Kahn's power in MKD, this is explained.
Both of them hace telecinetic powers, Ermac can control his powers bit more, but Kenshi is almost at same level with Ermac using telecinetics and he is better close up fighter.