Fischer wanted to play under a first-to-ten-wins format. This was the format of the first world championship match between Steinitz and Zukertort. The argument for this format is that it forces whoever is leading in the match to continue to play for a win. For comparison, in 1972, the Fischer vs. Spassky match was first-to-12.5-points. Therefore once a player is in the lead, they can just trade some pieces, draw some games, and coast to the title. This is (arguably) what happened in games 14 to 20 of the 1972 match.

The main argument against this is that it is impractical, and matches can be of unlimited duration. This can be dangerous to the health of the players, as happened in the [World Chess Championship 1984][2]. Furthermore, first-to-ten-wins gets more and more impractical as players get better (which they do, since chess theory improves over time). In the Steinitz vs. Zukertort match the championship was decided after only 20 games, but  [Fischer vs. Spassky][1] 1992 took 30 games. This doesn't sound bad but neither Fischer nor Spassky were top players in 1992, and earlier in the World Chess Championship 1984, in which one "only" needed to win six games to win, there were still only 8 decisive games in 48 games played. And if you are masochistic enough to pit two top chess engines against each other from the starting position on strong hardware & at long time controls, you could very well [end up with 100% draws][3]. 

In the lead-up to the 1975 match, Fischer had sent his demands for a first-to-ten-win format to FIDE. There were heavy deliberations within FIDE, with the federation ultimately voting against an unlimited match. Fischer refused to defend his title as a result. You can read more about the episode on [Fischer's Wikipedia page][4].


  [1]: https://en.wikipedia.org/wiki/Fischer%E2%80%93Spassky_(1992_match)
  [2]: https://en.wikipedia.org/wiki/World_Chess_Championship_1984
  [3]: http://blogchess2016.blogspot.com/2021/01/opening-selection-tcec-20-superfinal.html
  [4]: https://en.wikipedia.org/wiki/Bobby_Fischer#Forfeiture_of_title