Nobody here is going to be able to tell you why your wife doesn't want oral sex from you. People can postulate and provide insight, but only she knows the real reason.
As far as technique goes, that is a loaded question as well. In my experience there is a HUGE range of what women prefer. There are some basics like, wetness is better than dryness, and gentle at the beginning. Other than that, every woman seems to have certain things she prefers and some things she hates.
What one woman will beg for, another might slap you if you tried it. Like gently nibbling with your teeth or sucking hard on the clit. Some women love for fingers to be involved and can't orgasm without it; others find it uncomfortable and an unwanted distraction. Some like a stiff tongue directly on the clit, others want a soft tongue all around the clit. Some women seem to want a constant rhythm, while others want variety or a building crescendo.
You have to ask for individual instructions, start slow, follow her pointers, and be aware of non-verbal clues (breathing/panting, moans, hip gyrations, increased lubrication, and maybe a scream or two). To top it off, what she might like in the heat of passion she might hate at the very beginning. They're women for gods sake, they change their mind a lot, LOL!
Either way, I hope you somehow work it out with your wife. I know that I would not want to be in a relationship with a woman that hated receiving oral sex. It certainly allows me to enjoy the overall sex experience if she is having fun and since I am no great shakes in the intercourse department, oral is my best chance at that