There are two reasons why I think Tanks weren't used.
First off, Terrans ARE tanks. That is what made the Terrans awesome. Think about it, the Terran players used a bunch of new units, instead of using a Starcraft build. This makes more sense to show off what is new, isntead of just retreading old ground.
Secondly, and I think it a very accurate reason, is Tanks aren't as good against Zerg anymore. Sorry. They cost more minerals and gas. They do less damage to the smaller zerg units. They do while siege 50 damage (down from the 70 in SC) and only do another 50 to armored units. They're really only doing half of potential damage to light Zerglings and Hydralisks, with full damage to Ultralisks and Infestors. Roaches are armored to, but they're regeneration means it takes at least two to three tanks to take out a roach in one volly, or a tank firing two to three times. Splash is still good, but it means even less damage is spread out.
For crowd control you are spending more money to do less damage than you traditionally needed to. To me, it looks like tanks are moer on the "Battlecruiser of the land" type units, less flexible on what they are countering. Much more of a later game, large unit sniper. They'll clearly still be awesome for artillery and will still be used a lot, just not as flexible as they were in SC. If anything Nighthawks look pretty badass.
