Here's why:

Calzaghe-Hopkins was the first of the two fights to be put together. Calzaghe has never fought in the US, never on US PPV, most people in the US have no clue who he is. Hopkins is a known fighter, but he's never had a huge fanbase, and he's never been a big PPV draw. If this fight had been on PPV, it would have done very poorly. So HBO agreed to pay a big rights fee to air the fight on regular HBO with no PPV fee.

By the time Mosley-Judah came round, HBO had already paid the money for Calzaghe-Hopkins, had the Cotto/Margarito doubleheader scheduled on HBO for April 12th, and had the DLH fight scheduled in May, with another HBO card (with Gamboa) scheduled for later in May. They were all budgeted out for the spring, they didn't have money left to fund another free HBO card. So Mosley-JUdah had to go onto PPV or find another network like Showtime.