Isaac believed it was because Rebekah was barren. Therefore he prayed to God to allow her to conceive (Gen. 25:21).
A noble thought, but why did he not think that it was he who was barren? Was Isaac the ultimate sexist whose principle is cherchez la femme, always blame the woman?
Abravanel recognises the problem and uses other verses to prove that Isaac knew that in fact it was not he who was barren.
In Gen. 21:12 God had assured Abraham, “Through Isaac will your seed be assured”. Isaac was thus aware that he would have issue.
This was not the problem. The problem was, through which wife would the issue come?
This is why Sforno explains that though Isaac had been promised children, he prayed on Rebekah’s behalf because she was the woman whom he wanted to be the mother of those children.
In Biblical society it was accepted that a person could marry another wife in order to be able to have children; Isaac’s own father Abraham had done this.
But Isaac loved Rebekah, and he did not want any other wife. As a result, God made it possible for Rebekah to conceive, though she was still not able to have as many children as she and Isaac probably wanted.