In seeking an explanation for why several cities once adopted PR-STV, I sometimes encounter the following hypothesis. Among myriad goo-goo reform ideas, PR was unusually obscure, and that’s why it only happened in 24 places.
Obscurity could not have been the cause of PR’s rarity, and I’ll show you why below.
But first, a comment. I don’t need convincing. The National Municipal League included PR-STV in its model city charter from 1916 to 1964. (Does anyone else smell civil rights colliding with party politics?) The NML blanketed American municipalities with that PR-manager charter. If you believe the preceding, unsourced claim, ignorance of PR cannot explain its rarity among the hundreds of urban “reform” charters adopted by mid-century.
Others may not be persuaded. First off, I have no proof that the NML blanketed America with pro-PR propaganda. Second, PR has been off the agenda in this country, if you will, for most of the adult lives of those likely to have been paying attention. So I’m not surprised that most people think PR is weird. (How ironic, given the effort we spend teaching PR to undergrads, most of whom probably go away liking the idea.)
Alas, PR was not always as obscure as it is today. The animation below plots reform advocacy by the Proportional Representation League, 1893-1957. The intensity of shading corresponds to even breaks of a log-transformed count of PR advocacy events. I got the data by reading every issue of the PR Review over the period. Events qualify for counting if they’re major: no mock student elections or letters-to-the-editor. We’re talking conventions, legislative bills, whistle stop tours by PRL officials, meetings with charter commissions, and so forth.
You’ll also see some solid, black exes appear. Those represent the observation that PR came up for at least one local adoption vote in the state in question. I got those data from the same, as well as some books and articles. So feel free to replicate.

One other comment: it’s striking that near-adoptions are weakly correlated at best with the level of reform advocacy. That says something about the actors and motives in times and places where PR gets traction. More on that later!