Fischer wanted a return to earlier days in chess. Capa-Alekhine was an unlimited match to six wins. In some 19th century events, the rule was if the game was drawn, reset the pieces and play another. This is the rationale behind the request ("If this was good enough for Alekhine, why shouldn't I have it, too?")
My own opinion is that Fischer knew he was not in good playing shape, having not competed for three years, so he wanted a longer match so he could "play himself into shape," so to speak. (Something like this actually happened in the first Kasparov-Karpov match, where Kasparov was clearly inferior to Karpov at the beginning of the match, but built up his strength and technique through the games until becoming at least Karpov's equal by the end, if not superior.)
The Capablanca-Alekhine 1927 match showed the folly of unlimited matches (34 games, 25 draws) so that format disappeared from the chess scene until Karpov-Kasparov.
There have always been arguments over the World Champion's edge. Early champions got to pick who they would play a match against, leading to many good players never getting a shot at the title. The "London Rules" tried to put a stop to that, but it still required the challenger to put up a large sum of money, thus limiting challenges.
In the FIDE era, the champion often had a huge edge. Mikhail Botvinnik was champion a long time without ever once winning a match in defense of his title. He drew two matches, retaining the title by "draw odds," and lost all the rest, only "retaining" the title as a result of winning the mandatory return match the following year. In 1963 FIDE dropped the return match provision, and Botvinnik, after losing to Petrosian, stopped contesting for the title.
Botvinnik played with both "draw odds" and the return match, champions from there until the latter half of the 20th century only had draw odds, and that revealed the champions were often not as clearly superior as the record seemed to show to that point, as only Petrosian succeeded in defending the title (once, against Spassky, before losing the following cycle to Spassky) until Karpov.
I don't know why the unlimited match returned, but I suspect it was because non-soviet chess was becoming stronger, and might challenge their hegemony that the match rules got adjusted. And the first Karpov-Kasparov match showed us again why an unlimited match is a Bad Idea. And the return match clause merely added to the debacle.
The tradition of chess is that you have to dethrone the king, not merely match him, to take the crown. (Even the metaphors shout monarchy.) This is unlike pretty much every single individual sport on the planet; typically the defending champion has to engage all the challengers and prove they belong at the top (boxing being the lone exception I can call to mind ATM). So I doubt the challenger will ever be faced with a completely level playing field. (FIDE tried earlier, by creating a knockout tournament of short matches, but that met with a great deal of resistance from the players, and was dropped. And even in it, the playing field was tilted by seeding some players directly into the later rounds of the event.)
