LogFAQs > #970272536

Because back when Gen 1 came out, they kind of made up the Dragon Type for the singular purpose of being the super-endgame boss. If you want a Dragonite during the main game, you can use it, but you have to go well out of your way, either being a Game Corner Prize, or Fishing in the safari Zone, neither of which actually tells you anything about its value in battle. And add to that, Dratini is... horrible in-game. And even if you're training one, there aren't many pokemon with Ice-Type moves until you face Lorelei in the Elite 4, so you might not ever learn what Dragon's actual weaknesses are until its basically too late to do something about it.

So really, the first time you'll encounter a dragon type will likely be Lance at (what you think) is the very end of the game, with hard hitting monsters that resist whatever you're starter's (which is probably your best pokemon) biggest STAB move. Because it's basically making up a type for the final battle, you probably have no idea how to deal with it. It makes Lance a great "final boss" before the twist of a bonus rival battle at the end.

Slapping Dragon type on Gyarados and/or Charizard would hurt that. In addition to flying being a much weaker type and making the respective pokemon fit more with the power level of the rest of the pokemon.