Fear of rejection!
In their eyes they believe the person being asked has the "power". What they don't realize it's the "asker" who has the most power. He or she can move onto the next person and ask them out while other person has to "wait" to be approached again.
Basically there are two forms of rejection.
Direct rejection - Women use this one to turn down men who ask them out.
Indirect rejection - Men use this one by NOT asking a woman out.
Both can hurt just as much if you are interested in that (specific person).
"If it's not worth asking for then it's not worth having" - That goes for both sexes! The sooner you know where you stand with someone the better you can make use of your time.
One man's opinion! :-)