Because the current van and truck experience for me is a tracking update at 6am saying "Your package is out for delivery" and then it actually gets delivered sometime in the next 12 hours. As a customer that's a crappy experience. Hopefully flex will push everyone to up their game with regards to delivery experience. If vans and trucks are more efficient then they should win out in the long run because they can operate at lower margins.

Disclaimer: I work with Amazon, not on the Flex team. My words don't represent the company.