Maybe it's something to do with how 'bents look so different from an upright, particularly as almost every part of the bicycle is strictly controlled in racing. It's kind of silly how they're sometimes seen to be 'not a real bicycle', no doubt due to being so different and sometimes faster than an upright. After all, anything with two wheels propelled by human power is a bicycle- recumbent, upright, pedalled, rowed, hand cranked etc.
It's probably the same line of thinking that assumes anything that costs less than £1000, or doesn't have a 50-speed drivetrain, or aero this-that-and-the-other or weighs more than 5kg isn't a real bicycle either. There's far too much snobbery around for something as simple as a bicycle.