RCA connectors flex very little, and where they flex is not the same as where they make contact. The tiny contacts in, say, an HDMI or SATA cable flex a great deal. Flakes of metal would be a very bad thing there. Even USB has enough spring loading for this to become an issue, but that spring loading is on the device side, not the cable side, so nickel-plated USB male plugs would probably be fine. You're going to have to pick something else for the female end though, because of that spring-loading. Similarly, a headphone plug doesn't flex significantly because it is the jack that is spring-loaded, and nickel does just fine here.
Nickel plating is very sturdy when there are no tension changes in the underlying metal, but it utterly falls apart when there are. There are many nickel-plated musical instruments upwards of 100 years old that look wonderful (much better than silver, generally better than lacquer, and up there with untouched gold even though they are hardly untouched), so long as they have avoided taking mechanical damage. Once dented and repaired, the finish starts to fail in long, peeling strips and in smallish (1 mm scale) chips.
Nickel also has the drawback (as a musical instrument finish) of feeling "slick" or "wet", and of causing skin irritation in considerably more people than do silver or gold. Of course neither of these is a significant problem in a cable, but it does explain why nickel never really became a dominant brass instrument plating despite its durability (outside of drum and bugle corps, where they should be wearing gloves anyhow).