Another aspect of XP dependent upon edition is that in O/B/AD&D, different classes required different amounts of XP to go up a level. This was a balancing tool, which speaks to how balance in older editions was something that happened over the long term and was not necessarily a concern in a given encounter or even adventure.
though I wouldn't call it a good balancing tool. A tool should give you a metric. If, somehow in a pre-3e game, your PCs never died, by the time your wizard was level 20, everybody else was much higher. What level is your party? What level encounters should you use?
A useful tool is to have everybody's level mean "can travel together reasonably well" That way you can look at the party level, and eyeball what to throw at them. This was actually the design advantage of a level system, compared to others (like Shadowrun, how do you decide how tough to make the bad guys).
Note, I'm not even talking about how wizards suck at 1st level, and outshine at 20th. Just the fact that they used different XP scales REMOVED a useful metric for the GM. You can still have that with a consistent XP table, just by re-arranging what you get at what level (which in fact was already that way).
We put up with it in 2e, concerned that to change it would break the game balance. Its far more useful to know that if everybody's got the same level, they're the same strength (sort of). Without it, what did GMs do, eyeball it or go by XP total?
Making the XP tables consistent with the implication that all PCs of the same level were "good to adventure with" was a reasonable idea as it opened up a useful metric.
What actions earned XP, how much was given to establish a pace of leveing, that's a worthy topic in any edition.