From a technical/mechanistic perspective, what is it about XTOL that gives it such a favorable overall profile as a developer, such as combination of low grain, sharpness, and high speed? Is it the use of ascorbate, or is it the use of a phenidone derivative, or does it have something to do with pH or sulfite concentration? What about other factors? Or is it just some magic balance of factors that makes it so good?
Alan - to answer directly your question : Xtol is "so" good because many (most) like it so much!
It is relative : You also could have ask : Why is perceptol so far better in comparison to Xtol?
But then you possible would get many protests from the folks!
So let me state : the characteristics Xtol is specialized (advantage in regard of speed, sharpness,
edge effects, accentuated grain from a relative small size [compared with Rodinal], normal contrast
ARE to the same time THE characteristics what many photograpers WANT today!
Here Xtol is superior in comparison to ALL other developers! (in a most relative concern)!
At last Xtol is cheap AND "easy to handle"!
But if you like other characteristics from a developer you soon will have other statements to Xtol!
Ask people who are specialized with stain developers :" Why do you use that stuff - there is Xtol avaible and with Xtol you can get much better characteristics " - they will protest of course!
So Xtol is Allways such superior like the characteristics it serve are the characteristics MOST
photographers like to see at the same time!
A last example :
Meritol by Johnsons - a super fine grain developer superior in relation to Xtol !
But at the time it was on the market Meritol had exact the characteristics most photograpers wanted to see from a bw developer - regarding the advertising "for miniature films" aha

!
For what ? 35mm films

????
During this period Meritol was a best selling developer of Johnsons photograpers had the need of
enough resolution with 35mm films!
So today you have to look on developers Allways also in regard of todays films !
It seams to be so that Xtol there reached a best corelation with modern best selling films!
It you shot ISO 25 films (what is a niche inside a real minority) some (as stated here) find Xtol's
characteristics not absolute attractive

!
But who is using ISO 25 films

? The most speed from Xtol is not the central point then!
So Xtol is "good" if you are shooting ISO 400 films? One can say so!
BTW : What is the most demanted box speed with bw films at this moment ?
ISO 400 - and there you have it!
with regards
PS : Your question "Why is Xtol so good" could also be "Why is Xtol so popular"!
PPS : From the technical side Xtol is the " modern" D76 (with alternate agents)!