My understanding is that the RCA connector got its name from the company Radio Corporation of American which used phono plugs to connect their tonearms to amplifiers. It was used of course in equipment that carried the RCA brand and so it goes. Much like we refer to copy machines as Xeroxes even though numerous companies make them. But you won't find instructions for a Minolta copier say something like "to make 10 xeroxes do the following...".
To make a long story longer . Analogy of the why we call the plug RCA can be made with the computer PS2 mouse. IBM came up with the new connector in their PS2 computer line. After a while the industry picked up on it and started to use it. Before long it became the industry standard but we call it PS2 because of the copyright patent. And now it's stuck in our heads and we will for ever call it RCA plug and PS2.