Perhaps worth noting is that these failing literary magazines are tied to colleges? Perhaps without readily available student volunteers the magazines can't afford to operate.
As you mentioned, it seems that certain literary magazines are still doing relatively well. Maybe if more literary magazines published things that people wanted to read, they'd be better off.
Our journal explicitly rejects 99% of those submissions, just to stand out from the pack.
Is that true? e.g. http://www.mcwetboy.com/mcwetlog/2007/10/the_decline_of_the_...
The general take-away, it seems, is that genre readers are mostly a bunch of freeloaders; and more of them need to pony up some cash to compensate editors and authors for the enjoyment they're providing.
That seems broadly true of web content generally. There's no shortage of content of various levels of quality and no shortage of readers. But it's hard to get people to actually pay.