How very dare you. They're the superior cut and it's appalling side cut even exists. To answer your question though it's because that cut originates from New England and is used for "higher end" foods like lobster rolls. So they're perceived as fancier.
But they're just objectively better. Side cut buns are uneven which makes sense for a sandwich where you a thicker top bun but is utterly ridiculous for a top facing slice. Can you imagine a taco where one half of the tortilla was like three times as thick? Like look at this https://imgur.com/tF9uBp7 — absurd. That little flappy thing just falls apart. Also the flat part of the roll is at the bottom so when you top cut your hot dog can stand upright on the plate and not fall over. Typically the sides of top cut rolls are also flat which makes grilling/toasting the roll much easier. You get a delicious crispy buttery on the outside fluffy on the inside roll.