that was mainly for balancing reasons; it would have only be weak to a few types
Tbh, I really find this nothing but a myth - whenever the topic of why Gyarados or Charizard weren't dragon types are brought up. I mean, I've played the gen 1 games ever since it came out, and played it a lot.... and the whole type strengths and weaknesses were all over the place for pokemon with dual types in gen 1, such as:
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Charizard, being weak to ice... despite being partly fire type
(I remember my childhood, I used Charizard on Lorelei's jynx, and jynx knocked it out in one hit with an ice punch)
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Dragonite, being resistant to electric.. despite being partly flying type;
Gyarados, weak to ice;
Zapdos weak to electric;
Articuno weak to ice;
Venusaur, resistant to poison and weak to ground etc.., just off the top of my head.
And I didn't just have one gen I cartridge... I had all 3 - red, yellow and blue.. even in yellow, where a few tweaks were made (Charizard could fly in yellow, unlike red and blue.. though they didn't do the same thing for Dragonite, which was pretty stupid of them.. *shrugs*), the type strengths/weaknesses still remained the same.
Heck, I remember getting Pikachu to use thunder on Dragonite in yellow, and it didn't even lose 1/4 health.
I think the reason they didn't make Gyarados or Charizard a dragon type was because they didn't want the player to get an easy dragon type early in the game, like how they put Dratini as a rare catch in safari zone and the game corner - they wanted dragon types to be rare - to fit the idea of them being mythical, and both of them didn't fit that: you could evolve Magikarp to Gyarados in Cerulean, and you could evolve Charmeleon to Charizard by Celadon.