In 2006, Art Briles (now at Baylor) went 10-4 at Houston. In 2009, Sumlin won 10 games, and then his team went 13-1 in 2011 (Levine coached the bowl game, which Houston won).
They had other 10-10+ win seasons previous to Briles, so Houston has had some periods of success. It is not unreasonable for Houston fans to think that Levine was going to carry on the winning that Briles and Sumlin had. He was part of Sumlin's staff, so there was some continuity.
It seems the thing that stung Levine more than anything else was declining offensive production (ppg). He lost some good OCs (Kingsbury as Texas tech, etc.) and other offensive coaches, and his replacements weren't as good. This is not dissimilar to Purdue losing Greg Olsen and later Jim Chaney. Tiller's teams weren't as good after Chaney left.
BTW, Purdue has only had one 10-10+ win season in its history - 10-2 in 1981 under Jim Young.