Do you mean why it takes so long for a man to propose?
You have to consider some basic gender differences especially during our "formative years".
Imagine a little girl is given over the years a baby doll, a stroller, an Easy Bake Oven, a Ken & Barbie's playhouse to decorate, a princess dress to wear, and is read stories that have a fairytale ending comprised of a Knight or Prince who sweeps a young damsel in distress off of her feet and they live happily ever after. A (seed) of marriage and having/nurturing children has been planted in her subconscious almost from the time she could walk! On some level she's been (practicing to be a mother or wife) most of her childhood. During her teen and young adult years she witnesses weddings.
By contrast a little a boy is given remote control cars/boats/planes, games to compete with their friends, things that flash lights and make noises. Little boys don't practice being fathers or husbands during their childhood. They don't grow up dreaming about their wedding day.
Essentially most women have a 10 year or more head start on men when it comes to (thoughts of getting married) and having children.
A 22 year old woman fresh out of college might possibly be considering the idea of marriage while her male counterpart is just glad to be done with school. He's thinking about partying with friends, establishing a career, dating around and enjoying being a single adult.
The last thing on his mind is becoming his parents! The very thought of getting married, signing a 30 year mortgage, and having children for most guys in their 20s is like watching their lives flash before their eyes!
These days most men don't start thinking about marriage until they are in their late 20s or early 30s. A young woman in her 20s who has her heart set on getting married might do better to date a man who is in his early 30s.
There are two basic reasons why men do not propose.
1. Timing (He's not ready. He is happy being single)
2. She's not "the one". (Most men do get married eventually)
Statistically by age 44 over 85% of men have married at least once. I would imagine that (some of those) who haven't may be gay. Therefore odds are most men a woman dates is going to marry it just may not be her! That's my theory! :)