The reason Charizard will never be Dragon/Fire and why the type combination will almost never be used (Reshiram the only offender) is because there is only one Pokemon that resists both STABs - Heatran. Every other Pokemon takes at least neutral damage from both STABs, which is incredibly overpowered in most settings. It's one saving grace that keeps it from being the end-all be-all of Ubers gameplay is the fact that it's weak to stealth rock and other Dragons that are faster than it.

Charizard also has base 100 speed which allows it to speed tie with all of the BST600 Pokemon and most outspeed or tie potent Dragon-types in OU (Haxorus, Hydreigon, Salamence).