Why 1st and 2nd class but not fast or slow airplanes?
Concorde was built for speed. But it was costly too. People did not like it much. Reality was that they didn't want to get somewhere faster by paying more. People love to pay for luxury though. That is the reason different travel classes were introduced since late 1970s.

Cost of fuel and speed are NOT directly related. As you may know, Boeing 787 has increased fuel efficiency (claimed to be the most fuel-efficient), and still reaching higher speeds.