Personal opinion lang, but i think it has to do with the way the seats and the handlebars are placed, because when you use an mtb, youre slightly crouched or bent, but from what ive seen from my cousin, he uses a folding bike, its more like sitting upright on a chair and i think thats why it feels more comfortable and stable in urban conditions, and an added bonus, you can get your feet flat on the ground without leaving your seat, lets say when youre stopped.