Ramban: He figured that if Hashem would appoint additional leaders, the people would be less aggressive towards him and the leaders would help to calm the people down whenever they would come to complain. Moreover, it may well be that, when the elders prophesied and the people would see that they were certified prophets, they would sometimes approach them, thereby taking the pressure off him.

Seforno: He hoped that Hashem would provide him with others who would share the leadership with him - leaders whom the people would trust.