A combination of things. Salford have a coach with a history of poor man-management skills, Koukash is a volatile character and probably didn't feel like he was getting a return on his investment and Locke himself hasn't really played all that well and hasn't lived up to the billing as a high-profile signing, he's spent most of the time injured or looking anonymous on the field. Couple that with what seems to be a toxic environment at Salford at present with players looking to leave left and right and it's no wonder things didn't work out.
I think Locke has been badly treated TBH and wasn't really given the time or opportunity to prove himself. Salford are an absolute shambles off-field currently.