What killed downtown?

In 1950, the classic American downtown of Norristown, Pennsylvania, centered on the six blocks of Main Street, was the bustling commercial heart of central Montgomery County, and had been for over a century. With depression and war in the past, downtown merchants looked forward to an extended period of prosperity. It was not to be. By 1975, downtown's core stood largely shuttered and deteriorating, with 99 storefronts vacant and countless others lost to the wrecking ball, as first shoppers and then the merchants fled Main Street. What Killed Downtown? Was it... The Malls?-- Amazon.com.