Because when pressed on the question, both in the Matrix channel and on here, Rimu has admitted that the real motivation is that some people are voting 'too much' and have 'too much influence'. That was, in fact, the original reasoning brought up when he posted the graph in the Matrix channel, if memory serves, and was told by several people in the Matrix channel, including a fellow Fediverse admin, that it wasn't an 'issue' that needed solving. He went ahead anyway - implementing the voting limit before announcing it, and then only 'announcing' it as insofar as it was included as a minor detail in the update notes. It even took other Piefed instance admins by surprise.
"You are participating too much" is a shitty fucking viewpoint for any instance to have, and not one that's welcoming to users.