Someone with more draft understanding than me please help answer this question: How did Mitchell go undrafted?

His athleticism is elite, he has incredible speed and balance. He has a very strong understanding of run concepts (I’m consistently impressed with his ability to weave through traffic and find the hole in tiny spaces), and he had great college production. Teammates seem to love him and he has NFL bloodlines. The only knock I can see is his size and he went to ECU (although they have turned out stud NFL players in the past, too).

I just don’t see how his size and small school status could negate the speed, athleticism, and run ability. No one didn’t think he was the best pick available in the late rounds?