Also, many of the usual reasons a politician could drop out don't really apply to Bernie.
The big one is running out of money. After that, you want to make nice with other people in the party for the sake of your future ambitions. Bernie can afford to campaign as long as he wants, he's not angling for Secretary of Labor or anything, he knows that his campaigns have been by far the biggest platform he's had for his ideas, he's not running again and even if he was he's not concerned about getting endorsements for next time.
It's not just that Pete and Amy were dead in the water. They didn't feel like they were carrying the banner for a whole ideological program and they still have a long time in Democratic politics so it's good to get on the right side of the next nominee.