That could be because they were more and more comfortable with each other's bodies, or because they sought variety over the long haul, or for some other reason. Sometimes one partner wants oral sex exclusively, or a lot more than the other. Occasionally, one partner will go down on the other, but not vice versa. Usually, this state of affairs doesn't last long. Either they give it up, or the reluctant partner decides to join the party.
Many couples stress the special intimacy that fellatio and cunnilingus bring to their relationship.
I could understand why Steve liked me to do fellatio on him. I could see it was tremendously exciting and sensual and gave him a really strong climax. And I liked it when he went down on me, but I could never figure out what was in it for him. I love it. It's a wonderful, passionate shtick.