Why don't more people ride bikes for commuting.
In most spots in the U.S., for example, I suspect the reason is that towns (and particularly cities) tend to not be designed around walking and cycling; instead, they're absolutely car-centric layouts. Can take a good long while to get from point A to B, if attempting to go via some other method than a vehicle. I suspect that's a major reason, for many people.
As well, cycling (and walking) isn't an all-weather mode. A good percentage of people simply won't want to do what it takes to be warm and dry through all weather conditions. Going by car is too simple, by comparison, particularly when it's ugly outside.
And, I'm sure many simply live far enough away from work, in terms of the commuting distance involved and/or the routes necessary to get to the destination. Myself, I live in a spot where most of the routes away from downtown can be nasty in terms of safety, particularly at "rush-hour" times of the day. Which will certainly kill the idea for many people.
Over the decades, I've only met a handful of people that habitually commuted via bicycle. One colleague of mine did a ~55mi round-trip jaunt, often daily for weeks at a time. He had the right clothing, was strong and quick on a bike, and had a couple of key safe routes to travel for his trip. Me, coming from the other direction, I had only part of the route safe enough to travel without much worry; the rest of it was dangerous in the extreme, simply because of the layout of the roadways between my home and the company's facility. I'd do it, occasionally, but only using the light-rail (train) for part of the distance in order to eliminate the nastier unsafe sections of the route.
Would love to live in a place that was designed from get-go as a walking and cycling type of place, with roads following a well-planned but spartan network of vehicle routes that'd get close to shops and businesses but not right to the doorstep, then to have a people-mover type system of light rail cars that takes people into the business districts and shopping areas. Can't imagine the amount of work to retrofit existing towns (let alone cities). In the U.S., at least, there aren't many cities that are old enough to have roads and routes that pre-date motor vehicles, and most of those have long since been re-made to be car-centric in the extreme.
Wish there were an easier way.