When you go mech you can easily get enough viking to Zone the Mutas as long as you also have good turret placement and thors. Especially with the combined attack upgrade.
While you stay in your base and mine out while zerg controls the map. Nothing in that composition is mobile, and if you're caught out of position, which is super easy for mutas since they're so damned fast, you're done. If you move out, your base is screwed. Maybe I've missed the games, but I've yet to see vikings do shit against mutas except in very low numbers.
Supernova, and he's in the loser's match against Trap.
One little example from the actual match. Supernova killed Leenock's spire, along with the +2 armor upgrade. Then he immediately moved out with his entire force, but mech is so slow that with a little harrasing, Leenock had the time to rebuild his spire and produce a massive new muta flock that wrecked Supernova's mostly thor army. Even if he had known the spire had been rebuilt and started producing vikings immediately, he would have never had enough to do anything to this almost instant flock.