If He wanted to us to follow all of it and the oral law was most of what we received at Mount Sinai why didn't He tell us to write it all down?
Written Torah and Oral Torah are different in kind. Written text is static, an orality is dynamic. Hashem didn't want to hand us halakhah, He wanted us to figure our which path we will take to redeem ourselves. This is an aspect of what it means when it says "these [the positions of Beis Shammai] and those [of Beis Hillel] are the Ideas of the 'Living' G-d. And the halakhah is like Beis Hillel." If everything were text, there would only be one opinion, no human contribution. As it is, we are told halakhah on a meta-level, we are told how to extrapolate and interpolate new Torah from existing Torah.
This is also why the need to codify the mishnah was considered a tragic expediency necessary for Torah to survive altogether under Roman oppression. It's not that memorizing the Torah is better than having it written down, it's that the halakhah gained a rigidity that we would have been better off without.
If there were no Oral Torah, there would be no system of extrapolation to new cases like electricity of fax machines.
Some understand this as being a difference between the first Tablets and the second. R Yoseif Dov Soloveitchik (the first; ie the Beis haLevi, Derashah 17) writes that the first Tablets contained the entire Torah, even down to “a question a student will ask his rebbe in the last generation.” With the second Tablets came the concept of Oral Torah and the need for Torah study. They entail Hashem’s choice to make Torah less well known but more internalized into the people. Making the nation Hashem’s “parchment”. (Or as I put it, making the Torah a process, which then requires the Jewish People to actually progress.)
The Beis haLevi refers to the thought of Chazal which says that had we not made the Golden Calf, the redemption from Egypt would have been the complete redemption. That sin necessitated further exiles, a longer process to reach the ultimate redemption, And this is why the first Tablets could not exist in a post-calf world — for two reasons: First, because without the Torah being intimately tied to the Jewish People, our host nations would have co-opted it. And second, the unity of the people and the Torah would give us a self-definition that would enable us to survive as a distinct people.
Because it would be impossible to have written all the details of everything that had to be done. In fact there are many things that did not have words available to describe the details of the laws at that time. As an example, consider electricity or using a fax machine on the sabbath. It would have been impossible to write done every detail of every case of every law especially the cases that are on the boundaries.
Note the ORAL LAWarticle explains that no matter how much detail is given, there would be problems in understanding exactly what is written. As a result, Hashem created the oral law so that a person learning the Torah and Halacha would have to study with a rav who can explain the details and prevent misunderstandings.
Consider the story of Antignos Ish Socho who was misunderstood by his students Zadok and Baysus (Greek spelling Boethus) who founded heretical sects.
Thus, it is necessary to have an oral law so that the previous generation of Chazal can ensure the accurate transmission of the meaning of the Torah.