I'm going to say fear, fashion, social issues and family. I was terrified to get on a bike at first. Going fast was hard. Traffic was scary. Hills were hard. 6 weeks later I'm looking at some high end performance bikes. Depending on your "fear factor" everything adds up. My fear factor is a bad back. The thought of falling off and putting my back out for a year leaves me on the timid side.
Fashion:
I had to get over it. There are some women's clothing out there but most of it made me look like I was pretending to be skinny when I wasn't. I'm going to say that a lot of women are insecure with their bodies. Look at women's magazines and even something on the men's side of Sports Illustrated or Maxim. We can't compete with super models and we know it. If the world considers those women as a standard of beauty, we fall short. The same with women who are into sports. If we don't think we are athletic, we don't want to be laughed at while trying to be.
I don't know what the "average" woman size is but the industries are doing much better at fitting clothing to all sizes and even bike sizes. Most retail stores just don't carry them and have to be special ordered. Unless the guy says something, I don't think most women know there are bikes out there for them. I didn't. Had I know there was a women's 29er, I would have bought that first instead of my Hardrock.
I'll be honest..women like pretty things. If the bike is ugly, chances are she won't buy it. It sounds superficial, but it's true. There are some pretty ugly bikes out there and no woman wants a pastel colored bike when they go into a women's style bike. We aren't 10.
Social Issues:
Women are extremely social and we've been raised that going out alone is potentially dangerous. I do worry about traveling 5+ miles out of town by myself. If there aren't other women around to cycle with, chances are a woman won't.
If we show up to work completely drenched from a 5 mile ride, it takes time to get a shower, put make-up on (depending on how much) and look professional.
Family:
You can't carry 300 dollars worth of groceries on a bike. You can't take your kids to all the places they need to be on a bike. If your significant other doesn't ride, then it goes back to social issues.
Depending on the age of your kids, you can't just leave for a couple of hours. If your significant other isn't supportive than it's harder to get out and ride if he isn't willing to help.
We are the primary caretakers and are taking care of the needs of our families. It's not demeaning by any means, it just takes a lot of our time. I like being a Mom. I like staying home with my kids. I've found that cycling relieves a lot of the stress and I can come back refreshed and ready to handle teenagers. I don't think women realize how empowering a bike really can be.
The price of a bike falls under family. I'm guessing a lot of women see that money as bill money, groceries, clothes, and all the other financial needs that are there. Spending that kind of money on herself makes it hard not to feel selfish when she knows the needs of her family.
Hope that helps out some.