A lot of clubs in town sell it by the can, or at least they did when I lived thereRed Stripe is originally a Jamaican beer, so it became popular in Moss Side and presumably other areas that West Indians first settled in when they came to England. It was often pretty much the only beer on offer if you were at a West Indian club or a shebeen back in those days. So it then became trendy with students etc.
I didn’t know it was synonymous with Manchester though. I assumed it would probably be the same story in places like Toxteth, Handsworth, St. Paul’s etc.