Combinations. Amazon, smart phones, how kids hang out, poverty, giant stores like target and wal mart..It's a bunch of reasons that all hit against malls.
Malls haven't been the only hit over the decades. "Cruisin" is no longer a thing. Teens used to spend hours on nice nights driving up and down a certain stretch of road in nearly every city somewhere.
More kids used to ride bikes around for funnies.
Drive in movie theaters used to be huge.
Things always change and it's almost never just a single reason.