Because a great deal of the marriage is to manage the partner's feelings, expectations and keeping him interested in you, too.
To bluntly admit that a bigger cock is better or was better than what you have in home between husband's legs, could misfire in so many ways. A husband might feel unable to satisfy his wife in any way possible and the reactions could be worse. Detachment from the relationship, a runaway for porn or even leading him to look for another woman, hoping the new one would be then satisfied with his cock.
That is why most of the liberated wives or hotwives only open up about certain subjects, like what they love in other men bodies, way after the couple started to have sex with other people.
It takes time to open up about many of those things in sex, for most people. (wife).