Experience. The only way you start a kid with zero experience on national TV against your most hated rival in a season opener is because he's either miles better than your other options or you have no other choice.
He's not miles better (yet), and Enos has another choice (2, actually). It may be Jarren's time soon, but it's not yet.
And before everyone yells about Tate's experience, he has played in 6 college games, he has played in something other than total mop-up duty, and he's a year older aka more exposure to the college game. I'd certainly not call him "experienced", but he's got more than Jarren.