My conspiracy theory on this:

Before 1954, American tax code had the notion of straight-line depreciation. Buildings had a 40-year life, and each year, they depreciated 1/40th the value. This was changed to accelerated depreciation, which means a higher tax deduction initially, that tapers off over the 40 years. This meant developing a mall was a great tax shelter. Corporations could easily offset their income and then some by building a mall, then dumping it on another property management group before the depreciation became too low each year.

Tax code changed again in the 1980s to close this loophole, and when the malls were no longer good tax shelters, their value as investments plummeted. Also factor in that the popularity of this tax dodge meant way too many malls were built in the 1970s and early 1980s.

There's also a second round of this in that when malls went dead, developers would go to the local government begging for tax abatement if they de-malled and built some outdoor mall project to save jobs and economy.