I think people saw how big a draw Hatton was, saw that Calzaghe appeared to have a similar personality, was the "other" popular UK fighter at the time and thought he could get the same numbers.
It wasn't to be.
They forgot that the Calzaghe/Kessler fight was meant to draw 60,000+ people, and barely made 50k in Calzaghe's backyard. They forgot the poor ratings the match got in the US, and they also forgot that Calzaghe is far from a ratings winner for domestic tv.
Calzaghe used briefly had the equivalent of a network contract, where he'd fight on one of the big 4 tv channels at prime-time with a huge amount of advertising. Then he fought Manfredo, who despite his lack of comparable talent was somewhat of a draw due to the Contender.
The ratings bombed, and Calzaghe is now fighting on a subscription channel (and not even one of the "big" ones, but one you have to pay an additional fee to get). He just doesn't inspire the way that Hatton does.
I still have no idea how he won BBC SPotY... I could have sworn Hamilton would get it on the basis of following the great british tradition of nearly winning before falling apart when it matters.
Despite that, he's still apparently bringing 8000 fans over.