Why would someone want to fly on a US regional jet or older a/c when they can fly on a new jet blue 190 with live tv?
So they can earn frequent flyer status and miles useful for taking them to worldwide destinations, that's why. With B6, you get neither. B6 is great for the "always buy the cheapest fare and never fly the same airline twice" crowd. For frequent flyers who fly all over the world, B6 isn't on the short list of realistic options.