Short Answer: Because I wanted to.
Long Answer:
I cycled when I was younger, mainly around where I lived mostly XC. Got a bit boring as I only ever went around the local area and off-road was limited. I used to cycle on footpaths without much problem, only ever had one person say something - and at that point I was on a track where the tractors ruined it so bad they had to fill in the holes with bricks - so like I am damaging the landscape eh? Anyway - it got boring and along came a lorry that hit me whilst I was a passenger in a car... no more cycling or running (at 16-17). After a few years my back was recovering but I was finding it hard to get back on the bike which led me not to bother much and made it harder.
I did from time to time make the 1-ish mile trip to work on the bike in good weather, never really had anywhere to put the bike though and it felt like a hassle. I had enough money, it was only 1 mile and I was young - if I never drove to work I would never get to drive. And 10 miles a week wasn't going to break the bank (petrol was cheaper as well) and living sub rurally it would be hard not to have the car so insurance/ved/mot's were already being paid for. I did walk too, but this was harsh on the back.
Then a few years ago I moved, 10 miles away. I drove at first, it felt a bit sucky at first but never took that long and could be pretty enjoyable - could still afford it too without trouble. What I did though was get the bike out and explore the new bridleways which turned out to be rubbish as they were knee deep in grass and only really suitable for horses. Footpaths being my age (22) was slightly less unacceptable in my own mind too, although I did try a few but had many more negative responses - even though I stop to give way to anyone walking always. Roamed a bit more on the back roads, almost no traffic (was never worried about it but its a plus) and kept just going further - dropped some slicks on the bike and decided that the amount of time I was spending would be worth getting a road bike.
I first tried to look for a second hand bike for next to nothing but had no luck with that, but at that time a new neighbour moved in who was a bike mechanic at my LBS... I didn't know it existed! There is another one but I never really liked it in there so never considered this. I went along to the new one and was pretty impressed, had my eye on a few bikes, looked online and decided just to bite the bullet and spent £700 on a new bike - obviously most people thought I was mental.
At the time I only had a small thought about going to work a few times on it in good weather etc, and worked it out to be around 6 months solid riding to save that much in just petrol. At first I never did make it to work on the bike, but I did take it with me in the car and leave it at work - where I would go out and do 10+ during my lunch break 3-4 times a week.
After a while though, with the recession and such I was made redundant and had to look for work - this is what I think started me commuting ironically. I was confident I would find something else - so wasn't in a rush and spent my time "fannying about" (even spent 2 months abroad although that was already planned to be 4 weeks break from work). When I was back I had to go to the Job Centre to reclaim my taxes - I could still afford to drive but I decided to try and do everything cheaply so cycled there, the JC was right by my old work place so was the same as a commute. The brilliant thing about this was I could leave home 40 minutes before and always arrive there spot on - whereas in the car I would have to leave around 40 minutes before though as I wasn't sure about the traffic (15-30 mins mostly) and finding parking! They were also pretty good and let me park the bike between the 2 doors out of the way (although when my time changed the guard always had a major problem with it - another story).
Then 9 months later, less than the 10 months I calculated that my redundancy would technically cover (minus petrol costs etc) I got a job back at the same place - they made my boss redundant, expected him to come back as a contractor, he said no, I got his job. I negotiated with them, employed rather than contracted - they only wanted part time so got 15hrs over 3 days (2/5th my hours) for 500 quid short of my previous pay - jackpot! (hourly too so full time will be good, and manager position when things pick up, hopefully).
I was already used to making this trip on and off, so after a week of driving to settle in - I got on the bike. This official work commute was only last Feb. Last year I only made 4-6 trips in the car for various reasons from dropping off wifey or picking my sister up, running errands (which I always like to do all at once) and a few times because I was lazy or had no sleep. None of them rides off were due to
I don't think I am saving the world - and don't really believe in the Green movement, other than marketing and government taxation.
I do think that cars are not sustainable transport, even if we changed to electric cars 100% tomorrow, it wouldn't stop the traffic chaos in major cities. I used to drive a mile before... sure, but I didn't live in London or drive half a mile to get the shopping, or to the dentist - neither did I get stressed about people doing 40 in 60s etc, or speed cameras etc etc.
I see so much more whilst cycling - I even started walking more around the local footpaths and wifey would join me and enjoyed it. My goal now is to get her on the bike, baby squirrel is 6mos so we're busy with that - she is a bit scared of the road and cannot go that fast or far yet but in time (specially when I am towing baby squirrel in a trailer, that will slow me down

) - going to buy her a new bike to encourage her too

and then I want to do some small touring trips.
I might one day hack some electric bike up for lazy days, shopping, the wife to use etc. I will still continue being able to afford the car (a high powered sports car) as a luxury even though the fuel prices are going up like crazy - I can drive it pretty efficient and I don't hang around either (my mpg trumps most others on the owners clubs sites, no idea why they must have the worst looked after engines in the world).