RCA connectors were, and perhaps still are, used for speaker connections on low-end gear for many years. They take up less space than banana jacks and manufacturers probably have a bunch of them for all the other connectors so economy of scale applies. Not saying it's a good idea...
I am surprised locking banana plugs beat a spade lug, and that Maty's reference calls a spade lug a poor connection. That is not at all my experience but you do have to make sure they are tight. Not surprised SpeakON does best. I suspect the reason they've not caught on is cost and royalties to Neutrik plus customer acceptance since the consumer world revolves around bananas. Another connector I have no fondness for; often poor (high) resistance, tendency to wiggle loose, no positive capture mechanism (aside from the locking variety), manufacturing tolerance varies wildly, etc.
I have said for decades that an RCA is a bad idea. Aside from tolerance (range from fall-out loose to so tight you break the jacks on your equipment getting the @#%$ things on and off) and highly variable build quality (including such ideas as a plastic housing instead of metal so you lose shielding right at the connector), the idea of a connector that makes signal before ground and breaks ground before signal just seems stupid a really, really bad idea to me. Of course, many TRS/TS connections do the same or worse (like shorting signal to ground as you insert or pull the plug...)
But the market has spoken, and consumers pay thousands of dollars for cables with piss-poor (can I say that?) connectors on the ends, all the while hearing vast improvements.
Blah.