In the early days of bare-knuckle boxing, there was no limit on the number of rounds and so matches would be fought to a conclusion (i.e. with a knockout or tap out).
When John L. Sullivan made boxing under Queensbury rules with gloved hands popular, his matches were of a pre-determined length and the referee would decide the winner if they went the distance.
To regulate such results better, official judges were appointed to award points so that a technical winner could be determined.
[9] This changed though, following the death of lightweight Duk Koo Kim in 1982[2] after his fourteen-round fight with Ray Mancini.
[9] The following year on March 27, 1983, the first ever heavyweight title fight scheduled for 12 rounds under that rule was held by the WBC between Larry Holmes and Lucien Rodriguez.
[11] While the International Boxing Federation, which had recently broken away from the WBA, continued to hold onto the position there was no documented medical evidence to show a 15-round fight is more dangerous than a 12-round fight, they eventually voted to shorten their championship distance to 12 rounds as well on June 3, 1988.
You would think most boxing fatalities would occur in the heavyweight division since they are clearly the most powerful punchers.Lotierzo also argues that part of the motivation for a 12-round limit was not so much for safety, but to allow the matches to appear on network television.
[2] Previously, the timing of boxing involved 15 three-minute rounds with 14 one-minute intervals between each round, the preamble, and post-fight interviews—requiring around 70–75 minutes; in contrast, a 12-round bout lasts 47 minutes, which fits neatly into a one-hour time slot when pre- and post-fight programming and commercials are added in.
[16] However, by the 1990s, championship boxing had been almost exclusively become a premium pay-television (HBO, Showtime, pay-per-view) sport, meaning no commercials were necessary, and making that irrelevant.