Peter Woit posted some info about bickering behind the prize. Apparently Salam's work was unoriginal and undeserving. Salam thought that he deserved to share the 1957 prize for parity violation, and lobbied heavily to get it for something else.
Apparenly also Weinberg used to be close buddies with Glashow, but did not want to share the prize with him. So Weinberg was eager to credit Salam in order to cut Glashow out.
I am not even sure Weinberg was so deserving. His contribution was a short 1967 paper that was not hardly cited by anyone, Decades later prizes were given for the Higgs mechanism and 'tHooft renormalization, and they were arguably more critical.
A couple of comments mention that Salam was the first Moslem to win a science prize. I do not know if that worked in his favor, or against him.