Wages aren't everything. Lots of times I've had players sign for other clubs on less wages. Sometimes if you look at the news box that says 'Player X signs for Team Y' there's an indication as to why they chose to sign for that club, i.e "he was impressed with the clubs training facilities" or "is excited to work with *insert manager here*".
Also, he might just want to play in the championship rather than the scottish championship, as its a much higher standard. Although Rangers are a big club if he's got no emotional connection to them then the league really isn't appealing compared to the English Championship.