Just less of a track record (only 1 year of really high level play) and when he did explode onto the scene, it was largely against weaker opponents.
I'm on record that I love ZW as a draft prospect - I just don't get the talk about him going above Lawrence. Not a knock on Zach at all, but there's just so much more known about Lawrence. Multiple seasons of dominant QB play against a higher level of competition. ZW is a special talent and I'd love for my team to grab him (generally speaking, not my specific team - Chargers - since young QB is pretty clearly checked off at the moment), but I'd still take Lawrence over him 10 times out of 10.