That's an easy one, it's too expensive for high quality rhodium plated parts. Gear manufacturers will rarely spend the cash for pure copper connectors, and IME that's what's required for rhodium to sound good. In fact, I'd go so far as to claim that Furutech NCF parts are the only rhodium plated parts I've used that are extremely neutral. I think the question is why are so many manufacturers so cheap in their parts selection, even at the high end. But of course we can't answer your question precisely as different manufacturers may have other reasons, it's certainly possible some prefer other parts, but I'd bet 99% won't consider them due to cost or personal philosophy.
I think it's also interesting to note how DIFFERENT the FI-50 and FI-50 NCF sound! However, they have the same exact electrical contacts, but the NCF parts sound much cleaner and don't have a particular "polished" sound that most everyone attributes to rhodium plating. Well, the NCF plug doesn't have that sound, it's gone, and it leaves in it's place nothing... the closest thing to neutral we have available in AC connectors, imo. The sound may not be there with gold either, but it leaves a warm blanket over the music in it's place that maybe simply covers over the sound.
It's also interesting to note how different rhodium sounds when used to plate different base metals. Rhodium over bronze sounds MUCH different than rhodium over pure copper! But this is rarely mentioned, as if you can identify rhodium plating on it's own, but this is never the case, it's always used to plate other metals and it sounds very different in different applications.
This means the sound we attribute to rhodium may be something else, maybe it's noise in the AC or the sound of the base material that isn't being masked? In any case it's not rhodium that makes for that sound if the addition of NCF has removed it! I think in audio we tend to draw conclusions from limited evidence as we can't experiment endlessly, and unfortunately this means sometimes we draw incorrect conclusions.