Literally half the roster are fire classes kinda cringe now that i think about it.
Fire mage
Lava shaman
Destro warlock
Sun paladin
Wildfire bomb hunter
Fire breath dragon evoker
And sometime even warrior
How has this been acceptable for so long.
Is fire the only archetype we can come up with
If stretching were a sport, you’d be the gold medalist.
Guess they wanted to compete with the champ.
Fire mage is fire based.
Elemental shaman uses fire, lightning, earth, and occasionally ice, wind, and water (generally, the elements)
Destruction warlock uses a mix of fire and fel magics, really doing a “hellfire and brimstone” vibe more than just fire.
Paladin uses divine magic, not fire.
Wildfire bombs aren’t really fire either, more improvised explosives laced with poisons, pheremones, and spiky bits.
Evoker uses fire and arcane magics together, leaning more towards arcane in single target, fire in groups.
Warrior occasionally can deal fire damage but they have never been fire themed.
I’d give you fire mage, half points for destro and devastation? Out of 39, that’s… Not a lot. Like, 5% roughly.
I’m surprised holy priest didn’t make the list. Giggles while looking at holy fire icon
Fire is amazing! I would like more fire spells, not less! If anything, what surprises me is that we have two specs about frost
Expand your vocabulary.
Often there’ll be mobs using a mix of fire and warrior abilities but I can’t see or recall any fire effects. 
edit: WAIT, I do recall the two legendaries using fire abilities. 
There’s glyphs that make you catch on fire when you charge/enrage, which is probably what they’re referring to.
To answer the OP’s original intent: because fire does big dam and big dam good.
Odyns Fury is also a talent warriors can choose to do AoE fire damage
Fire is a major factor to all of them very disingenuous pretending this isnt the case.
You can change the color of the fire all you want its still fire. Sun fire, green fire, exploding fire, dirt and fire mix. Im surprised azeroth doesnt have the universes best fire department
Don’t forget Firebreath Chili
Why I don’t have a fire spec?
Even the warrior idiots have some fire abilities!
Sorry, walking freezer spec for you 
well… freeze can technically burn…
Turns out, fire hurt.
I have a sneaking suspicion that there’s also a lot of people using pieces of metal to bonk each other with, too.
Having a fire move, doesn’t make a class, fire based… 
Totally. Plus you’ve got that helm enchant. Fire. Then that ring. Fire.
It’s just fire everywhere.
Nothing could possibly ever amount to this level of
Why are half the classes fire based?
Well fire results from a sudden release of the electromagnetic force caused by the oxidation of a fuel resulting in heat and light. It can be very useful in battle.
We need a spec that centers around steam magic/steam power, and then we have the cycles of water!
Solid (Frost DK, Frost Mage), liquid (Resto Shaman), gaseous (conspicuously missing…)