Why Rhodium?

1. Poor conductor
2. Hard which makes for reduced mating contact surface.
3. Only used in audio
4. Uneducated consumer

I am sure there are more reasons.

Current estimates are a leading edge semiconductor plant costs $10-20 Billion (that's 9 zeros). Even a large geometry analog fab is high 100's of millions. I.e. price is really no object. You will not find one rhodium plated connector in that plant.

It’s interesting that folks who actually have a credible story / testing to tell about their experience with rhodium connectors vs gold, etc all agree that rhodium is a superior connector metal / material with least impact to the energy and openness of the music signal. 
 

those who don’t have any experience to speak off think it’s a bad connector material. And will never try it. 

Rhodium is not only a poor conductor but IMHO too hard.

To get a good grip on an outlet or speaker or any other contact you want a soft metal.  Something that will deform under pressure, allowing for serious grip, especially for AC plugs, sockets and spade connections on speakers.  Pure copper or gold plated copper will absolutely outperform Rhodium for reliability alone here.

Rhodium, as pretty as it is, is really a terrible choice in this manner. 

Since rhodium is indeed a fairly poor conductor, my assumption is that it is used to plate silver so as to prevent the dulled appearance of silver oxide...EXACTLY like cheap gold plating is used on myriad copper products to control oxidation. The latter's "bling" has rationale in that copper oxide is NASTY...not conductive at all. But silver oxide IS conductive, so plating it with an inferior conductor like rhodium for "bling" is a decidedly bad idea.

OTOH i cannot comment on rhodium's "sound", as the plating is so thin, and its purpose silly, that I completely avoid it. Benign silver oxide needs to be wiped off only perhaps when you pull out grandma's cutlery for company!