Well one of the reasons is he's just not that great. And another is the drug test issues at the combine.
Shirley he filled out the paperwork to be evaluated by scouts prior to declaring for the draft. Had they told him he should go in the top 3 rounds it seems odd that he wouldn't get drafted at all. If the scouts said he would be a late round selection, why would he even declare? Either this guy has a lot of idiots around him giving him very bad advice or there is a behind the scenes reason he left early.