Here are the main reasons for all the things you listed, including the ones already noted.
Ho-Oh, Reshiram, (Chandelure and
Blaziken on some tiers)-Uber
The first two are pretty self explanatory, as they have over base 600 stats and movepools able to rip apart most OU pokemon. Chandelure has the shadow tag which is pretty much a free kill on a proper switch, then he leaves when a revenge killer comes up. Pursuit jumped up in use partially because of him. Blaziken is mainly there to stall out and give speed to slow, powerful pokemon, making for a harder kill and a strong, bulky sweeper.
Infernape, Heatran, Nineatales, (Blaziken and Chandelure here,)
Volcarona-OU
Infernape and Heatran aren't weak to stealth rock unlike most fire types; Ninetales is mainly there because of its ability to put up instant sun, and volcarona might get a double stealth rock weakness, but it can boost its already high special attack and speed with Butterfly dance and go for a sweep.
Arcanine, Darmanitan, Houndoom,
Rotom-H, Victini, and Vulpix-UU
Arcanine and Darmantian are strong physical sweepers, but are outclassed in speed(Darmantian), stregnth(Arcanine), and typing by infernape. Infernape's typing is perfect for an offensive pokemon, since he can get STAB fire and STAB close combat.
Houndoom Has pretty good speed, and makes a near perfect counter to Chandelure, but is still screwed by passive damage and weather. There are also a ton of fighting types to exploit his dark typing.
Vulpix obviously wouldn't be in OU since ninetales is in there, and putting it in NU full of Chlorophyll pokemon isn't a good idea, so it finds use in UU. (And even then it's still suspect)
Victini-This one has been suspect for a bit, but that C-Create, along with his coverage moves and abilities, make him a very dangerous pokemon.
rotom is strong, but outclassed by Chandelure in terms of fire moves (and gets Shadow Tag), and is also hit hard by stealth rock and passive damage.
Charizard, Typhlosion (You forgot
him)-RU
Charizard, though still suspect, was recently moved up from NU as you know. He can be a strong pokemon, but is still outclassed physically and specially by several other pokemon. (This assumes no stat boosts, since any pokemon is dangerous with Quadruple attack. I'm just looking at Charizard without setting up.) He's also one of the few fire types to get a double stealth rock weakness, meaning he'll lose half his health upon entry, meaning a Belly drum would be much harder to pull off. He goes for a dragon dance, then something could force a switch on him and make him waste it. He can be powerful, but almost every set that isn't outclassed by something else requires some kind of setup before hand, whereas something like Infernape is already a pretty strong pokemon from the start. A Dragon dancing or belly drumming Charizard is powerful, but you have to burn a few turns to get those going, and you have to be able to take on anything that comes. One pokemon able to check you forces you to switch and give up what you worked for.
Typhlosion-He can hit hard with special moves, and is one of the few pokemon able to learn Eruption. Unfortunately, it's based on his health, meaning stealth rock puts him at an offensive disadvantage as well as cutting health. He also has a pretty shallow movepool, most of his good moves being physical, which he isn't able to take advantage of well. With sun support, he can be dangerously powerful, but that could be said about just about any fire type.
Rapidash, Flareon, Moltres, Magcargo,
Entei, Camerupt, torkoal, Magmortar,
Emboar, Simisear, Darmantian-Z,
Heatmor-NU
Entei is outclassed by Arcanine, keeping him out of the upper tiers. Flareon lacks a reliable STAB that's physical, Moltres is somewhat outclassed by Charizard, and gets double SR weakness. Magcargo, Torkoal, Camerupt, and Emboar don't do well since fire types don't make good walls or tanks most of the time because of their common weaknesses. Rapidash is also outclassed by Arcanine, Victini, and Infernape. Magmortar is outclassed by Volcarona and Typhlosion, and still holds as a respectable choice sweeper in NU. Simisear is outclassed by pretty much any other fire type, and Heatmor has a pretty limited movepool.
Remember that Smogon and several other sites base their tiers mainly from Usage, not all effectiveness. Something will only be moved usually if it has a limited number of counters (that aren't situational) or if it has too much use, such as the case with Misdreavus in Little cup. The lower tier fire types can still be useful in the upper tiers, it's just that they don't see as much use-it's not saying they are bad.
I always thought Fire was a necessity
because of Ferrothorn and Forretress.
Fire types are on the same boat as ice types: They can't do much defensively, and their type is screwed by the main pokemon out there in addition to stealth rock-But their STAB is what keeps the game alive. Ice pokemon don't see much use in high tiers, but ice moves are crucial to keeping dragons in place. Fire types do the same thing with steel types. Due to this, a fire type pokemon isn't necessary, but it's always a good idea to carry an fire type move. Sand and rain also keep most of these guys out of OU, as they are screwed in both. With the sun being the second least common weather in OU behind Hail, they're pretty limited in their advantages. There's not a quota we have to meet for each type, just look at poison and ice pokemon.
