Because he wanted an idiot like you to ask this question on a forum where the answer is already obvious.
Same reason Holyfield, Michael Spinks, Bob Foster, Ezzard Charles, Billy Conn, Chris Byrd, James Toney, Archie Moore and all the rest moved up. $$$$$$$$$$$$$$$$$$$$$
Money. If there was more money at 175, he would have stayed there (no matter if he had to starve to make weight).
There is no greater prize than becoming the heavyweight champion. Any fighter who thinks they have a chance at becoming heavyweight champ would do it.
Money, and he didn't like starving himself making weight. Michael Moorer @ LHW= Edwin Valero Awesome record, limited opposition.