It's hard to generalize, here, because it seems to be more of a case by case basis. But I think some men have this idea that women don't enjoy performing oral sex, because the women they've been with either didn't enjoy it, or said they didn't. And maybe it's true that some women don't enjoy performing orally, or receiving orally either, for various reasons. It could be they've been sexually abused, or have been taught sex is bad; maybe they have body issues and other kinds of insecurities--it just depends on the person.
In my experience, also, many guys are not attentive with their girlfriends or wives enough to really care about what they like sexually and so on. They're only interested in themselves and having an orgasm. On top of that, if you have a girlfriend or wife who experiences a negative sense of body image, then they may be uncomfortable with sex. And so it's important to be attentive, and giving with your partner, helping your wife or girlfriend feel accepted, comfortable in her own skin, not so you can get what you want, but to help create a healthy environment in order for her to explore the sexual and sensual side of herself without fear or guilt. The women I've been with often find that they like something sexual that they never thought they'd like to do. My last relationship, the girl I'd been dating had never received oral sex before and found that she liked it quite well... and was more than willing to allow me to do that, as it's something I like doing quite a lot.
