The marriage happened because at a Chicago Convention where both Lee and Shooter attended, crowds asked Lee if he would have Peter marry MJ, to which Lee said he was okay with that and then put Shooter on the spot, to which he said yes too and that then got picked up, leaving the two crowd-pleasers with a shot at earning a lot of love.
So the request was for MJ specifically to marry Peter and not Spider-Man marry just anyone.
It's unlikely Gwen would have gotten the same backing and popularity, not if Lee wrote Gwen in the strip anyway like he wrote the one in the comics.
The newspaper strip came out in 1977 likewise. That was one year after 1976's Superman versus TASM crossover written by Conway in which MJ and Jameson were the supporting characters who accompanied Spider-Man cementing them as the most famous and representative supporting characters in the mythos. Lee was quite aware of the optics of that, so he might well have decided to go with Mary Jane anyway in his strip.