Ole Miss is still in the SEC, a dominant player from there will have a high enough profile
I agree that Nkemdiche will be a high draft pick regardless of where he goes, and Ole Miss does have some impressive defenders in the league (Willis, Hardy, etc.), but LSU will offer him more exposure.
LSU currently has 41 guys in the NFL, nine of which are defensive linemen. Ole Miss has 21, four of which are defensive linemen. LSU will more than likely have a record number of guys drafted this season, will have more nationally televised games, and has two vacancies at DE. He'll be good anywhere, but it doesn't get any better than LSU for his position.