It's not a cheap sport to really get into. With running, you buy a pair of $100 shoes and you are good to go. Biking I'm guessing is around $300-500 (or more if you want better stuff), even if you are starting with a used bike, if you consider bike racks, bike shorts, bike shoes, pedals, helmets, blah blah blah. Most 20-25 year olds don't make a heck of a lot of cash. I know I didn't, I got my first serious bike when I was around 25 (more than 10 years ago), and the $350 investment was not trivial for me. I really would not have been able to do that a couple of years earlier. I thought I would pass out a few years later when I dropped $800 on a road bike.

Also, I think many people, and maybe women more than men, are intimidated by the idea of riding a bike in areas with cars. If they do not have groups to ride with, and do not understand the rules of the road, it could be well, scary. A lot of women - and we all know who they are - have that eek, not little old me attitude. I think that goes away a bit as you get older and try things. Boys are raised to jump into physical things. Girls are often raised to be more cautious.