Because they think Pie is the better fielder.
It doesn't mean that Scott sucks. It's not like he's putting up Dunn numbers in the OF. Even if he played all year, at worst, he'd be something like a -8 or -9 fielder. Hardly a lead weight to his overall value. Hell, just from moving to LF from DH would probably equal out the loss in defense, in terms of overall WAR.