Some guys just can't - they psychologically are not comfortable enough to do it. It's not rejection. It (in all likelihood) has nothing to do with you. Some guys treasure and respect their girlfriends too much to do what is considered "dirty" to them. The only time you see that act is in pornography (usually in gross gritty detail). It can send the message that only slutty girls do that, OR only a**hole men do that.
Unless this is a major concern in your relationship, I wouldn't worry about it. Don't take it as a form of rejection.
I agree with slow_healer. Guys can have the same hang ups with oral sex as a woman has. Do not take it personal. Maybe when you have been together for a long period of time he will welcome a change in you sexual relationship.
Don't let this bother you grnpurple. I have never been able to get ejaculate with oral sex. No one I have ever been with has been able to make me ejaculate that way. I don't necessarily agree that it is a psychological problem. Mine is that I need a fast pace to ejaculate. If I have sex slow it takes me forever and a day to ejaculate as well. Maybe it just takes him a while to orgasm and most woman don't like to give oral sex that long. Believe me he wishes that he could. I bet if you let him masturbate while you are giving him oral sex he could orgasm for you. It's worth a try.
Thank you all for your comments and suggestions. I do appreciate the insight.