There's your answer--cuts into Fetzima's market share.
Same reason Lilly didn't push atomoxetine for depression--would've interfered with Cymbalta's rollout.
I think they intentionally wanted to stay away from the label indication for MDD. Especially if that opened up an easier path for Fetzima. Tons of people with fibromyalgia are offended at the suggestion they take an antidepressant. You can give them Savella and say that it isn't an antidepressant.