Because cycling is not always enjoyable. My motivation comes and goes, I can ride 3-4 times a week then nothing for a few weeks, odd ride then a break etc.
I don't enjoy going out in the cold or bad weather that's just a chore to me. After coming home from work o don't always feel like dragging the bike out and likewise on a weekend I might not fancy spending 3-4 hours riding when I have other things to do.
So why a lot of nooks quit is because it's probably an impulse buy and once they get bores instead of waiting for motivation to return they will sell up. They will probably buy another a few months later.