Having done a bunch of audio shows from the other side of things, I can say that, while maybe a company like Magico can pick anyone they want, these pairings are frequently a product of financial contributions to the cost of the show. We've shared a room with fancy cable brands a bunch of times, and honestly have never felt pressured to defend their need. The people who want to notice the cables do, and the rest don't. The cable noticers mostly aren't interested in anything else in the room and they talk to the cable guys and not me.
Outside the brands that are both high price AND high volume, there are a lot of mom and pop operations and guys with day jobs and retired engineers and things like that in the high end audio industry, and even if they are selling expensive gear, they are operating without much room for error and doing these shows really adds up. Also if you just make speakers and no amps or amps and no speakers you have to partner up. The cable manufacturers are (mostly) totally dependent on others to show their gear.