I know plenty of people who are within cycling distance, but haven't chosen to cycle because they prefer other options for transport and fitness.
Cycling comes with certain inconveniences that it takes a certain enthusiasm to overcome. You have to own a bike you're happy to use. You have to be fit enough for the distance you need to go. You have to plan out and learn a route. You may have to be confident/experienced/macho enough to ride among traffic and navigate big multi-lane junctions. You have to be willing to shower and change at your destination, or not do so. You might need to buy lights or bags or even different clothes. You have to be willing to get wet in the rain. You may have to figure out how to store your bike at both ends.
I agree not everyone will be able to change to cycling, of course. Reading to Paddington can be done in 30 minutes by train, but takes 4 hours by bike - and some people have disabilities or other things that keep them from cycling. But I'm confident there are plenty of people who can rise to the challenge despite not having done so before.