I think they were right to keep Hulk for last.
Just think of it: if they had released him early, people might have said "no way, I can't ever collect them all this way, I'm not even going to start my HT Avengers collection".
But now, they pick up Iron Man or Cap, it gets the ball rolling, they do start to consider "buying them all". Once they get to the last one, Hulk, there's no going back, they simply must have that last (expensive) one, too.
Good marketing strategy on HT's part...
"Everything has an end.
Except sausages, which have two..."