Seeing that WotC doesn't mind giving every full caster access to Fireball through evocation adept, why have they still not made a specialized fire mage subclass?

This gotta be the most common blasting playstyle any new player would naturally go for. You make a caster, you want to blow shit up, and naturally you gravitate toward fire. Yet we still don't have a proper subclass that actually supports being a dedicated fire mage. The only real attempt was the UA Pyromancer Sorcerer, which was never published and was shit anyway.

And I'm not talking about simply having access to fire spells. Just because you have fire spells doesn't mean you are a pyromancer. You can make a lightning-themed build and pick Tempest Cleric or Tempest Sorcerer, and there are similar ways to support other damage types and themes. But fire doesn't have a single subclass that actually supports fire as its core identity.

Light Cleric is mostly a radiant-focused subclass that happens to give you fire spells, so it's basically the same thing I mentioned above. Evocation is good for blasting in general, but it's not specifically a fire subclass. Its features work with basically any AoE damage, and it's generally good for blasting at higher levels, regardless of whether you're using fire or something else.

I know fire damage is generally better than other damage types, but resistance to fire is also more common. and you could take Elemental Adept as the feat tax for specializing in fire anyway.

We're talking about one of the most obvious elemental fantasies for a caster, and probably one of the first blasting concepts a new player would naturally want to play. So why has there still not been a proper Pyromancer subclass?

Not a subclass that just gives you Fireball. Not a subclass that happens to have fire spells on its list. I mean an actual subclass built around being a fire mage and making fire do interesting shit.

How is Fire damage generally better than other damage types? It's a somewhat common Resistance/Immunity.

the damage numbers are usually higher, most common outlier is fireball.

so it is balanced out, higher damage, more resistances.

Fire damage is generally worse than other damage types with the exception of poison.

There is a circle of Wildfire Druid subclass that shares some of this vision. With the feat, you could pick up even more fire based spells as well

Lightning Bolt also does 8d6 damage, and Spirit Guardians often deals more damage over time, with notably far better upcasting.

Circle of the Wildfire Druid and to a lesser degree Spellfire Sorcerer.

Spirit guardians is cleric exclusive and has other conditions

it is easier to hit multiple with a circle than a line.

There’s not a lot of elemental specialization period outside Draconic Sorcerer and a rare stint with the likes of Storm Sorcerer or Celestial Warlock. Even Tempest Cleric is a bad lightning/thunder specialist.

I think it’s an intentional design choice to not mentally pigeonhole casters into taking just 1 damage type and getting completely throttled by certain encounters. Same reason we don’t have a lot of subclasses that play super hard off conditions like Poisoned or Frightened.

how is wildfire considered a fire mage? it is all about your summon.

If that is true, why not make a subclass to make it better?

Fireball isn't an outlier in any way except being slightly easier to target. If you make a third level spell that deals ONLY damage, it should be dealing one of the four or five common elemental types and it should be 8d6.

But there are ways to not get throttled, ignoring resistance, sometimes even immunity in rare occasions, applying conditions, etc...

While OP is wrong about fire being a good damage type in this regard, it is a good damage type in that there are actually spells of most levels that deal templated damage. It's also a very cool concept and is common enough in other media. Any implementation would have to be careful not to step on wildfire druid, evocation wizard, and draconic sorcerer, but it should definitely be possible to make a fire mage subclass in D&D without coloring outside the lines.

1. Most common resistance
2. Most common immunity
3. Single-damage-type gimmicks rarely work; force/radiant/lightning/necrotic/cold are the only ones with good representation because they're also the least resisted damages.
1. Ignoring resistance and turning immunity to resistance gets rid of \~45% of the struggle; immune things are still a pain, and there are a few creatures with super-immunity to fire - they heal instead of taking damage.

A pyromancer is a sorcerer with transmute spell, or they're just a caster choosing fire spells. Try a genie warlock (efreeti) multiclassed with bladesinger, taking the fire giant feats, AoA flavored as thermal leeching,