[1] https://www.cnn.com/travel/article/standing-up-airplane-seat...
[1] https://www.cnn.com/travel/article/standing-up-airplane-seat...
If I were to guess it’s probably because of aircraft evacuation and/or safety standards.
Or maybe it’s just not straightforward to make bunk beds fit with potential need for more aisles, or dealing with the cylindrical shape of the aircraft.
The man says desperately trying to convince anyone that this is a good idea. Also he's talking about 'short-haul' flights then mentions 3 hours which is not exactly a short time to be stand-squat-sitting on a bike seat...
What has changed is yield management — airlines have got much better at flying with as few empty seats as possible. So the main thing that has gone down to allow prices to decrease in real terms is your chance of getting an empty seat next to you.