Processed foods and fast food aren't the choice of the masses because they're good, but because they're available, cheap, and designed to make you want more.
It's not about 'did you enjoy it', but about 'would you like another one...here, here's another one right here.'
I assume I saw it on the McDonalds movie or maybe not, but the success of McDonalds (and eventually all the others) besides the above, was simply consistency. Back then, you could order the same meal 10 times and get it (subtly, but noticeably) 10 different ways or qualities. I assume some of you older guys remember a place you could frequent and depending on who was back in the kitchen that night, it would influence your selection.
But all McDonalds did was make their food the same, exact way every time. And that's all that was needed to spread like wildfire. It's an institution. It's the first fast food joint a small town gets when it's big enough. It's on every interstate exit. It's astounding.