There was a greater uptake of cyclists in London this week due to the tube strike. And hadn't a lot of them pulled the cheap and nasty out of the back of the shed and tried to ride it - there were poorly inflated tyres, rusty chains, noisy bottom brackets and all manor of creaks, clonks and squeaks all round London. I also saw a lot more poor riding including going through red lights and squeezing up the inside of buses etc.
No wonder that they would all revert to their usual mode of transport after that experience. Trying to ride a few miles on a poorly maintained, heavy and totally unsuitable bike (as in full bounce mountain bikes with knobblies - and they didn't know they could take the bounce out) is an uncomfortable experience and not one most would be keen to repeat.
They simply cannot understand the experience of riding a decent bike, with properly inflated tyres and no squeaks or clonks - and the fact that our legs don't feel like theirs did the morning after!
So the answer to the OP is, as in most spheres, lack of understanding. They don't understand our experience and can't believe anyone would "endure" (from their perception) something so uncomfortable and tiring on a daily basis.
Nobbers the lot of them!