Ego and wealth.

He could have driven or had a 'chauffer' drive them, but Kobe had a huge ego and thought traveling by car is for little people. Even with fog and weather advisories against flying in a helicopter that day Kobe's massive ego combined with the wealth to accommodate his petty demands created a life threatening situation.

The pilot warned Kobe, but Kobe called him a cuck and demanded he fly them to the event.