It's a combination of two things: -The Tarver fighter -The fact Antonio Tarver is not Joe Calzaghe He was basically waiting for Calzaghe to get old.
He wanted to pull off a feat, prove himself, he knew Tarver was a sitting duck after the Rocky film and wanted to take advantage
His hero Sugar Ray Robinson went from MW straight to LHW, and lost, and he wanted to the same thing. (not to lose but to do what his hero didnt) (watch the Tarver-Hopkins countdown by HBO) Tarver was considered far more dangerous than Calzaghe at the time, and jumping 2 weight classes to dominate his opponent after being written off. Tarver also generated more money. And B-Hop started his career at LHW aswell.
He was coming off two losses and therefore did not have the clout to fight the man in the SMW division, Joe Calzaghe.
Also B-Hop chases RJJ for years, and Tarver was the man who finished RJJ, so he wanted to go up there and clear up the RJJ loss.
At the time when Hopkins made the move, Tarver was a much bigger name than Calzaghe (Tarver came off a Roy Jones win and a Rocky movie while in that period Calzaghe for the first time stepped up and finished Lacy. Hell, Calzaghe was fighting someone named Sakio Bika in the month Bernard was fighting Tarver). The 168 division simply didn't offer anything around that period compared to 175. Calzaghe really became a big name when he fought Mike Kessler in a geat fight (which was a year after the Tarver fight).
Hop wasnt killing himself to make 160. He made 156 for the DLH fight. He also started his career at 75 and chose to drop down 2 divisions to 160.
cus its an unglamourous division. same reason why duran skipped 140lbs at the time it was unglamorous.