God was working things out in Abraham's and Sarah's lives, and He had His own schedule for bringing them to the point of development where He could use them the way He wanted to.
Is this unusual? No, it is not unusual at all. We all operate according to time schedules; we all set priorities about things. A teacher has 180 days in which to get across that year's knowledge to the student for him to advance to the next grade. Teachers operate in a similar way to God.
God may have said to Himself, "It will take Me so long to bring Abraham and Sarah through a series of training programs until their faith, convictions, and character are to the point that I can really use them. Then, at the appointed time, Sarah will have the child."
Abraham and Sarah had to cooperate with this. They had to yield to God's way and to exercise the faith that they had. If they were really tuned into God, if God was the center of their life, they would see in this process of time and experience through which God was putting them (what we call tests, discipline, or chastening) that positive and good progress or change was taking place in their lives.
John W. Ritenbaugh
Conviction, Moses and Us