Because he has too many counters in RU. Examples:
Rhyperior will laugh at Charizard. Seriously. The usual set of Rhyperior in RU is
Rhyperior @ Leftovers
Ability: Solid Rock
EVs: 252 HP / 28 Atk / 228 SpD
- Stealth Rock
- Rock Blast
With full investment in Special Attack, Choice Specs, and by using Hidden Power Grass, Charizard can 2HKO Rhyperior. Not very useful when Rhyperior outright KOs Charizard with Rock Blast, or can just roar it away.
Blastoise counters Charizard well. Also Blastoise is my favourite Pokemon, it's a win win! The usual RU set is as follows
Blastoise @ Leftovers
EVs: 252 HP / 252 Def / 4 SpD
- Rapid Spin
Now Charizard can at best 2HKO Blastoise (the more usual Timid variants have only a 5% chance to 2HKO, so keep that in mind) with HP Grass, while Blastoise can simply Scald, or threaten Charizard with Toxic. Now in a 1v1 confrontation a Modest Charizard can defeat Blastoise but the more usual Timid variants will most likely not.
Aerodactyl literally flies over Charizard. Usual RU set is as follows
Aerodactyl @ Life Orb
EVs: 252 Atk / 4 SpD / 252 Spe
- Stone Edge
- Aerial Ace
Stone Edge will 1HKO. And Aerodactyl outspeeds. "But Sensei, what if Stone Edge misses?". Well Charizard's most common and powerful move (Fire Blast) will only 2HKO and has shaky accuracy itself.
Jolteon shocks Charizard by making it seem like a weak little bird. Usual RU set is
Jolteon @ Leftovers
Ability: Volt Absorb
EVs: 80 HP / 252 SpA / 176 Spe
- Baton Pass
Jolteon outspeeds and 1HKOs. And Thunderbolt has perfect accuracy. Bye bye Charizard.
All in all, Charizard has too many flaws to even be considered for RU. A quadruple Rock weakness (Stealth Rock says hi) and above counters are only a small selection of RU Pokemon that can easily beat Charizard. You also mention how Charizard works well in UU, and I do have experience with UU and let me tell you he doesn't. Blaze is a poor ability as it requires you to be nearly KOd to be useful and while Solar Power can be good the usual Life Orb Charizard will be taken a ton of health every turn also factoring in the boost. Drought is also banned in RU so you need a Sunny Day user or to give Charizard that move, which weakens it considerably.
Hope I helped!