One catch is that Journeys revealed Ash's Pikachu inherently has the Gigantamax Factor as it revealed its G-Max form early in Journeys.
And Pikachu with the G-Max Factor are expressly unable to evolve.
In this case it technically counts as a retcon since G-Max wasn't a thing until Gen 8, but this basically explicitly rules out any prospect of Ash's Pikachu ever evolving, as this essentially for all intents and purposes means Ash's Pikachu is explicitly unable to evolve. Not that it would ever want to in the first place, since it was established very early in Ash's story that it didn't want to evolve. But now that basically gives an in-universe catch that ensures it never will even if it hypothetically did change its mind because it outright can't.
Unless, by some chance, Ash one day used Max Mushrooms to undo Pikachu's G-Max factor. Which would undo the catch. But that basically, possibly unintentionally or intentionally, solidified it by making it outright unable to evolve in a way.
Eh, different continuities aren't necessarily bound by the same rules. G-Max Pikachu being unable to evolve in the games and Ash's Pikachu being able to Gigantamax is more of a happy accident, if anything - there's a mechanical reason the NFE GMaxes can't evolve in the games so it's not like that was done with the intention of it paying off in the anime. What I'm getting at is that if the writers wanted to have Ash's Pikachu evolve in the anime I don't think they'd let the fact that he was able to Gigantamax stop them.
And I would actually make the case that it weakens the original arc of Pikachu deliberately and consciously not wanting to change to say "well, he couldn't have done so anyway even if he'd tried". It feels like fixing a plot hole that never was and it's a trope I feel like I see in a lot lately (the whole "they were inherently special all along" thing - it's so ubiquitous in long-running media).
Unless you want to theorise
reeeeeeally wildly and say that Pikachu's stubbornness and strength of will was so immensely powerful it metastasised inside him and gave him the Gigantamax Factor, which... actually squares the circle in quite a nice way now I think of it. And isn't all that out of line with the way willpower has been portrayed in this universe.
Is it ever established why some Pokemon don't want to evolve?
I noted this a while back as I rewatched the original series a few months ago when they started being put on Youtube.
- Speaking of evolution, am I crazy or do a disproportionate amount of Pokemon we meet in the anime have an antipathy or outright dislike for their evolutionary relatives? Pikachu doesn't want to become a Raichu, Bulbasaur refuses to join in the evolution ceremony, Squirtle's terrified of Blastoise, Meowth loathes Persian. Lt Surge's Raichu even seems to find its unevolved self contemptible. It makes for an interesting parallel to the idea of a child hating the thought of growing up (which may well have been the actual intention). Pretty sure of all Ash's Pokemon, Caterpie was the only one who actually actively wanted to evolve; for all the others which did it was basically a happy accident. I know why most of those Pokemon don't evolve for meta reasons but it's funny they made it into an actual thing.
Given the target audience, I think they were leaning into the idea of "I don't want to grow up" quite hard, but for a lot of Ash's Pokemon in particular it often came off more as "I'm fine as I am, thanks, and I won't be told by you or anyone else that I have to change". There's quite a strong message of non-conformity there, particularly with Bulbasaur refusing the call to join others of its kind in evolving at a certain time - it's literally not following the crowd, and holds its ground by saying "I'll do this when
I want". Which is basically in tune with Ash's personal philosophy of doing things his own way.
I don't think this reasoning makes sense for the anime verse because it's trying to go for a slightly more realistic(?) bend. Or at least a less gamified one. Levels stop actually existing at some point and TMs are just flat-out missing. Pokémon just learn moves over time via general training (or the occasional dues ex machina) and there really isn't a good reason why evolution would/should play into that.
Size could technically be a factor but this also doesn't really seem to functionally matter for combat performance in the anime. Larger Pokémon can still be fast. Hell, Ash's Sceptile gets the jump on Speed Forme Deoxys!
Evolved Pokemon in the anime are shown to be
generally stronger as a rule of thumb but there's nuance to it; a lot of the time we're shown that impressive-looking Pokemon actually... aren't. Ash's Charizard, for instance, is big and strong and typically wins its early fights by virtue of that (in Johto, Misty calls Ash out for using it to easily win against smaller unevolved opponents) but it's poorly-trained and disobedient and when it gets compared against multiple others of its kind it's shown to be much weaker than they are. In one of the episodes I've just watched Ash and Misty get into a dispute with three trainers and Tracey observes that each of their Pokemon is lacking in some way: one is vitamin-deficient, one has poor colouring, and one needs more exercise.
And even though levels don't exist in the same fashion as the games, the advantages to delaying evolution are still shown to exist in a similar manner: in the Bulbasaur episode, for instance, it defeats Team Rocket by learning Solarbeam, something it's only able to do thanks to delaying evolution. And, of course, as in the games, some Pokemon are outright
unable to learn certain moves after evolving; Brock explicitly calls out Surge's Raichu as not having learned the speed moves it could only learn as a Pikachu (which is true of all the games up to SwSh). That doesn't make his Raichu weak - obviously not, since it defeats countless challengers' Pokemon with ease - it's just a flaw in its method.
Granted, it's never really explained in either continuity
why exactly delaying evolution results in quicker acquisition of moves - I mean, the gameplay reason is obvious, but I mean a lore reason. Whatever the case, it's fairly evident that age =/= evolution, and that maturity and wisdom are qualities unevolved Pokemon are just as capable of having as evolved ones.