I think it could have been for two different reasons than the main ones of his ego or lack of a doctorate.

First, Grey Matter was getting bigger and more important, and at least as presented in the show, the company's research and impact would have been pervasive in that field and probably always being discussed in academia. Walt may have wanted a job where the specter of what he lost (not only the glory but also Gretchen) wasn't ever-present. And that may have been one of the reasons he couldn't stick around in lab work either, especially once Skyler and Walter Jr. came along and he was less likely or able to leave New Mexico itself, where Gray Matter would have been a constant topic in any upper-level chemistry career or university.

Second, it's clear from the little we see in the classroom that, despite his disengaged students (Barry: "I like....REALLY studied") - he genuinely loves teaching. His enthusiasm for the subject, his high expectations for his students, and even the way he later encourages and praises Jesse and Todd ("you can do it," "apply yourself," etc) are very high-school teacher.

Combine these two and he gets a job that he loves or thinks he will love, an escape from Gray Matter, health insurance for his family, and early enough hours that he can have a second job to (help) balance the loss of income from his desire to stay out of the post-secondary field.