If 2+ runners have the A standard and you don't, even if there's a 99% chance that you'll blow up and crash badly if you try to run the A standard at the trials, there's a 100% chance that you won't go to the Olympics if you don't.
I realize that some runners worked extremely hard to barely meet the B standard and might not last more than a few laps at A standard pace, and I can understand why they might prefer to run as well as possible for their talent level and just be grateful to have run in the trials. Especially if they're young, they might chalk it up to a learning experience with plans to have improved in 4 years.
Still, it seems in every race there are a few people who don't have the A standard but are close. What's the reasoning behind not going all out in the trials, even if it means dying and DFLing if you don't succeed?