He didn’t want to write them. Gnome Press owned the original series and he didn’t get any royalties for them. In 1961, his current publisher Doubleday acquired them and for 20 years he told them no to writing more Foundation books. In 1981 Doubleday said they would pay him 10 times his normal rate and that is when he wrote Foundation’s Edge.

This was all printed in the front of my copy of Foundation and Earth. Titled as “The Story Behind the Foundation”.

I plan on reading the Robot series next.