The mall was dying by the 80's, there was a sharp decline by then (I recall seeing numerous malls going vacant in the 90's, around the country).
The things that drove mall popularity (especially things like large, enclosed, air-conditioned space), were no longer novel. Most cars were air-conditioned by then.
I'm sure there are many other factors, like the growth of free-standing single-vendor buildings (so construction and management costs must've changed).