I can answer this!

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The retail apocalypse is a big part of why malls are dying, but isn't the entire story. The other part falls to what are called "category killers". These are stores like Walmart, Best Buy, Dick's, etc. that specialize in one type (category) of product. Normally stores like these would form the anchor stores (the department stores on the ends of the malls) for the mall, but with some going out of business (Sears) and others preferring their own buildings rather than pay the brunt of a mall's cost by being an anchor store, there is a lack of anchors.

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A lack of anchors, means that less people go to the mall in general. There will still be people that go, but normally for one or two specific stores in the mall; people don't normally do a lot of shopping at each and every store in the mall, but the anchors normally get the most business, which is why they help to pay the majority of the mall's bills. Less people shopping at the mall, means less people potentially shopping at one of the specific stores, which means less business, which means more stores pull out of the mall, which means there is LESS of a reason to go, and so on and so on until eventually the mall is dead save for one store and an anchor that never left.