If Mariota is a can't miss then why are the Titans passing on him too?

With the majority so convinced that the Bucs are taking Winston then why are the same folks so convinced that the Titans are willing to pass on Mariota and go with Williams, Fowler or trade down?I get they have Mettenberger but even Glennon was better as a rookie and yet all signs point to the Titans passing on the guy some here think should be a Buccaneer.