Avraham Avinu and Sarah Imeinu were born into idol-worshiping families and an idol-worshiping society.
They are the models of the BT (Baal T'shuva - in our time, the term is most often used for someone Jewish who grew up in a non-religious family/environment and became Torah observant at a later point in life) or the Ger Tzedek - people who discover for themselves the G-d of Israel and His Torah.
They, who change from their early lives to become Torah Jews, embracing the Torah's values and mitzvot - its way of Life. Yitzhak Avinu is the first FFB in Jewish History. (Frum from birth - born into a religious family).
He did not make his own discoveries of the One G-d and had much less direct interaction with Him (at least from what we see is recorded in the Torah). Avraham could have written an inspiring, exciting autobiography.
Yitzhak probably wouldn't have written such a work, especially because the more exciting parts of his life were patterned after his father's life. But Yitzhak serves a different role in inspiring us, his descendants as well as Avraham's, throughout the last three and a half millennia.
Personal note (to make the point): I grew up in a religious, observant home. I had a Jewish education from earliest childhood through Yeshiva University and ever since. But I know Baalei T'shuva. And I have been inspired by them. BTs seem to come by an excitement for Torah and Mitzvot that many FFBs are missing. But that's the way it is. That's the way it must be.
Some people have certain experiences. Others hear about them and learn from them. Yitzhak Avinu is crucial to the Chain of Tradition because he was a solid Jew. Because he was brought up by an Avraham Avinu and a Sarah Imeinu. Rather than remembering idol worship in his youth, he remembers a lifetime of knowledge and faithfulness to G-d, of chesed.
As for all of us - some have the 'Avraham' type of background. And some of us have a background more like Yitzhak's. And Rivka Imeinu? Look at her background and look at what she became. She was worthy to marry Yitzhak and to take over the role of Sarah, straight out of the house of Betuel and Lavan. Pretty amazing. And we all know of people today who "went the Rivka route".
Our Avot and Imahot are not storybook characters. They are true models - each one different - for us to emulate. The Torah repeatedly points out how much Yitzhak was and acted like his father. But they were different. And the strength of our Chain of Tradition is that we are not all alike. But dedicated, nonetheless.