> he was working as an instructor (not a professor)

No he eventually became a full professor too.

"He began work in 1949 with a $5,000 salary(equivalent to $68,000 in 2025), maintaining this position for several years. By 1952, however, he was making more money as a writer than from the university, and he eventually stopped doing research, confining his university role to lecturing students.[g] In 1955, he was promoted to tenured associate professor. In December 1957, Asimov was dismissed from his teaching post, with effect from June 30, 1958, due to his lack of research. After a struggle over two years, he reached an agreement with the university that he would keep his title and give the opening lecture each year for a biochemistry class. On October 18, 1979, the university honored his writing by promoting him to full professor of biochemistry."

I've read his biography. It was definitely intentional - and of course making a living by writing was a big factor. But he just didn't like the academic environment or his colleagues.