I read up more and it's actually many many non-leguminous plants it attacks too. Seems like basically anything. There are native pests like that for pretty much every plant that grows in large populations so I highly doubt it found a new niche. It's success specifically seems to be due to just how much of a generalist it actually is

A more likely, though surely still incomplete, explanation may be due to the fact that the insecticides you need to repel it are broad spectrum and would kill beneficial insects just as well. Most other bugs that fulfill this ecological role have very targeted insecticidal sprays. So they end up at a comparative disadvantage to this generalist

A more likely explanation for their sudden decline imo is that there was probably some virus or other disease that evolved to pray on this now-ubiquitous bug. The bug also has a very particular relationship with its symbiotic gut microbes that seems really important. Such a virus/disease could also have targeted this symbiote. Either way I think the more likely explanation is that this generalist was countered with something that learned to specialize in it