is one of those funny concepts in gaming. Generally, you want each player to have an equal amount of enjoyment from a game, but there's no metric for "fun". You can try to make sure everyone has equal "spotlight" time, but that may or may not be what every player wants.
The more wildly characters diverge in ability, the more lopsided things can become (see Shadowrun's "Decker Problem" where you have a character who underperforms most of the time, then basically dominates large chunks of play when their "moment" comes up, because nobody else can do what they do).
XP advancement was intended to be a balancing factor, but it never really worked- 2 Thief levels are not equal to 1 Fighter or Cleric level, and while they may be worth quite a bit more than one Wizard level at low levels, at higher levels the Wizard gets increasingly more per level (in the form of spell slots) and has the rate of progression completely shift, further muddying things.
But outside of the largely useless at low level Wizard, you could squint at most characters played at the levels most people tend to play at (1-7) and say "yeah, these guys are roughly equal to one another", and that was good enough for the time.
Homogenization of xp tables unfortunately aggravated things because now a level in a class really should equal a level in another class, but there's no way to make that happen without also having homogenization of character classes, which some people really dislike. Many people want a unique play experience (and presumably, one that doesn't suck), and a lot of pressure is put on the DM to provide that.
Which ends up with an Avengers problem pretty quickly- a highly-trained super-soldier from WW2, an actual thousands of years old God armed with artifacts, a highly-trained super spy, a guy who turns into an unstoppable rage monster, a dude with trick arrows who never seems to miss, and a super-wealthy hyper-intelligent Artificer with bad impulse control who the DM has given up on trying to set limits for and so they can have any upgrade or gadget they feel like- having an adventure that gives everyone equal spotlight time might be difficult at best.
You could make sure that over the course of a campaign consisting of many adventures, is to make sure that everyone has equal spotlight time, which might be easier, but it might mean that long stretches of play might have a player feeling left out.
It doesn't help when games are a bit lop-sided with regards to how much combat/exploration/lorefinding/NPC befriending is actually going on either.
A lot of this speaks to the kinds of campaigns that were played in the early years- characters adventuring in different places at different times, rarely forming permanent bands of heroes, with lots of NPC hirelings and henchman filling critical support roles. Sometimes it's one guy playing D&D by himself, other times it's a small army of adventurers taking on a dungeon- or even competing groups!
Especially with high character turnover, you could easily have new characters teaming up with a local legend, the only 11th level Fighter in the entire campaign! Or a group realizing they need a specialist and asking if they can team up with another player's 14th-level Thief/M-U!
When I was young, I engaged in a lot of that "Wild West" kind of gaming, and "balance" wasn't really a thing. Some characters were just better than others- they had better stats, better levels, better magic items- but it generally kind of balanced out somehow.
Now I'm older, time is more valuable, we only meet to game for a mere 10 hours a month, there's only one DM, one adventuring party, and any imbalance between characters becomes really noticeable and undesirable.