As has been discussed, Uceny's decision to stay down was an emotional one, based in the deep-seated parts of the brain determining how you react when everything appears to have gone to pot. The injuries she suffered were not of the immediately acute debilitating type (like a torn ACL or bone break), they were after-the-fact collision injuries, like a hard tackle in soccer, you know, the ones those USA soccer women get up from every couple of minutes for 90 minutes or more, but hurts for a week or more, and needs treatment. She didn't immediately hold her injured back or hip, she pounded the ground in frustration, crying.
Simpson is just tougher. Her immediate reaction was to cover up from spikes, and then get up and kick like a Motha. All that crazy stuff happened, and she still finished within 5 seconds of the winner. She has my respect.