To many (most?) men, oral sex is much more interesting that intercourse. Where intercourse is thought of as "making a baby", a blowjob is more sexy in that it requires the female to do something just for the sake of sexual release for her partner, and it allows for the male to closely observe all the action. Also, there is less worry about premature ejaculation. There's a lot of psychology involved.
It maybe more mental the physcal. You may be enjoying the oral so much that when it comes to intercourse your mind may tell you it is not as enjoyable.