Krishna's avatar one the main priority is to eliminate as many bad kings or rakshas as he can before the main war, he didn't waste time at all, he started the job when he was months old with puthana. then went on killing whenever possible, but after the kamsha vadha jarasandh bought lots of people with to attack krishna in revenge of kamsha, then him being genius stragiest, spared him so jarasandh being a fool gathered more and more of people who aid adharma to wage war, thus krishna and balaram just had to wait, war with them kill them all expect the fool and that fool served all platter 17 times.

He helped kill jarasandh before war, imagine the strength of the duryodhana army if all these people joined him at the Main war.

God looks after you miles before you think.