Answer: Your wife simply needs time to make love to herself. During masturbation, she likely can relax more, be selfish, and as a result experience increased sexual pleasure, and perhaps stronger and more numerous orgasms. This has nothing at all to do with you. Respect her need for time for herself and privacy. Men frequently masturbate even when they have access to sexual partner, why shouldn't women?
You might suggest that you masturbate together on occasion. You then may learn to experience orgasm at the same time. Having orgasms at the same time may be a bit overrated. If you are having an orgasm it is hard to be aware of your partner's. This is different from when her orgasm triggers your own. Try to enjoy the pleasure you do experience.
I would say the majority of women are like your wife in regard to vaginal orgasm. Please read the pages about Vaginal Intercourse and Vaginal Orgasm for more information.