Numbers game. Surplus of skilled forwards in Hartford above him made it tough for him. Kreider, Miller, Lindberg, Kristo, Hrivik, etc. As a result, he was competing for a lesser, more defensive role. St. Croix is not a defensive forward by any means and I guess ultimately management decided he would be better off getting top line & PP minutes in the ECHL than on the 4th line in Hartford.
Now Kreider, Miller, and Mashinter are up. Thats a lot of offense (by AHL standards). St. Croix was killing it with Greenville (granted, only two games) and now there's an opening for him in Hartford to get some legitimate minutes. Obviously the AHL roster hinges on so many factors; send downs, injuries at the NHL and AHL level, NHL players getting healthy, etc. But if St. Croix impresses with Hartford then they'll find a way to keep him there, I would think.