Because most batters are right handed, and when right-handed hitters hit a baseball, it tends to go more often than not to the left, the left field home run line tends to be significantly further from home plate than center or right field. So a throw from deep left field is a very long throw, a Hail Mary, a bomb coming out of the sky, hard to see, hard to follow, hard to predict. Layers of meaning in this metaphor.
It's lonely being an Litfag.