Combo of things. He is slow afoot, left-handed and doesn't have a strong arm. As a LF, slow afoot lessens his ability to throw runners out because he has to turn-around to make a throw. He ain't no ballerina. The lack of a strong arm exacerbates the issue. A right-hander in LF would almost always be in a position to make the throw back into the infield right away. In RF, or even CF, Grossman's problem would be less prominent. I'd call Grossman a classic #4 outfielder. Not good enough to start, good enough for pinch-hit, DH and occasional OF duties. If there's a MLB-ready OF in the minors, Grossman is an easy cut.