Some people simply are talked into believing certain kinds of gas are better.
Some people are procrastinators, or on a trip. They should have bought gas yesterday, or at the last cheap place, but now they don't have a choice. They need it right now, or run out... and the expensive place is the only one available right at the moment.
All the pumps get better gas from the tankers at one time or other. Some people happened to get some better gas at a name brand one time, so they think it is always going to be this way. And they don't think about it any longer, except when they get a noticeably poor tank full.
Some people listen to their spouse or their good buddy.
Some people fill up because of convenience... it's right around the corner from work.