I suppose it all depends on what sort of tastes you like. There's an Italian place in my town that is nicer than a lot of places, and it is popular, but both of the times I went, I hated the food I had. The chicken Alfredo was too sweet and so was the one marinara dish that I got. I don't really like sweet when it comes to Italian.
I like Olive Garden, but I don't go very often because I'm a huge Parmesan cheese lover. They don't have Parmesan cheese in shakers on the table, they put it on your dish for you and you tell them when to stop. If I really had them continue cranking that little contraption until it was as much cheese as I really wanted, they'd be standing there for a good two minutes. You know the size of the typical Parmesan shakers at restaurants? I empty about half of a shaker onto my pasta dishes. I don't want to make my server stand there grinding half their shaker onto my pasta. So I don't go very often, and when I do, I'll get an appetizer as a meal, something that I don't need Parmesan on, like that chicken flat bread thing.
It's also money. More expensive items tend to automatically cause people to think it's of higher quality than less expensive items. And it's also the atmosphere of the restaurant that makes it seem nicer than others. Olive Garden is very pretty, they serve good wine, they play calm music as opposed to rap, rock, country, or hip hop, and the decor is rather nice. I'm sure that if other restaurants, like Subway, changed their decor to, say, chandeliers, serve wine, serve you your dishes to your table on nice platters, raised their prices, took down the wall menu and had actual menus, etc., they might be perceived as a higher quality restaurant.
I might also add that I bet more people go to Subway than Olive Garden, because more people can afford Subway and it's faster. Not everyone can afford to go out to a nicer restaurant. And a lot of people go to Subway for the health reasons, although they don't necessarily actually get the healthier choices. Subway has a healthy rep, so a lot of people think all of their stuff is healthy and will help you lose weight, when really it's only a few select options that are on the healthier side.