Disagree that Trubisky gives them the best shot. Seeing parts of the games this week, Trubisky is Trentative personified - very indecisive and his vision only covers a few yards past the LOS.
My guess is that Tomlin would rather know about Pickett now than play safe and extend the uncertainty. Much as the Steelers suck on OL, Trubisky hasn't shown the ability to play around those limitations. It's tough on Pickett, very tough but that's how things have been working in the NFL - been thrown into the fire with little runway to acclimatize to the NFL