Why didn't Ash use Charizard against Gary?

I'm talking about the first battle they ever had (right after Ash won Orange
League).
-Gary was his sworn rival.
-Ash should logically want to beat him more than anything else.
-The fight was one on one pokemon.
Yet, instead of sending out his strongest pokemon at that point, he risks
battling with Pikachu. And don't tell me he did it for fair play sake. Ash
was enough of a jerk to send Charizard right off the bat against a novice
trainer (Casey) as we saw a few episodes later, thus crippling her confidence,
yet, in one of the most important fights of his life (Gary), he takes the chance
with Pikachu.

Misty, Brock, and iris accuse Ash for not thinking before he acts so that could have been a reason why he did not send out Charizard and another reason could be because Pikachu was his starter Pokemon and they have been through alot together. And as for sending out Charizard against Casey, I had to read the episode guide since I did not remember really well why he sent out Charizard and it just says he finds himself reminiscing over past events before sending out Charizard.

I would assume its because Pikachu is his first Pokemon and Gary, iirc, poked fun at the pair of them in a previous episode or two, so the victory would mean a lot more since it would show how far they had come as a pair, as compared to using any other Pokemon, and prove Gary wrong about them.

I think its because Pikachu had been with Ash from the beginning. And Ash thought that Pikachu's experiance could help him win the battle and Ash possibly could of thought that Pikachu was his strongest Pokemon. Since it did take down a Dragonite.

Because Pikachu is his main Pokemon.
Pikachu had just beaten Dragonite so Ash thought Eevee was fodder and Pikachu could beat it with ease but it didn't happen which shows how inexperienced trainer is still Ash when Johto saga begins.

The question that I would ask is not Why didn't Ash send Charizard? Instead I say why would Ash send Charizard in this situation? it had no particular type advantage in this matchup.

In hindsight since Gary won it is easy to say that Ash may (or may not have) done better with a different Pokemon. However, in all fairness was there anything in the present situation to indicate that it would go badly? I really don't believe so.

Pikachu is Ash's most experienced Pokemon, as others have pointed out it had recently won Ash the title of Orange League champion by taking down an undefeated Dragonite. Yes, Charizard is a fully-evolved Pokemon so in that sense it was stronger, but Pikachu's levels were almost certainly higher than Charizard's at that time because Charizard had only been in a couple of battles by this point, while Pikachu had been with Ash from the beginning.

Ash was still new to training Charizard, while Pikachu was the Pokemon he had the best synergy with. It was not unusual behavior of Ash to pick Pikachu, he always picks Pikachu if Pikachu is willing and able, Pikachu is his oldest and best friend and usually does come through for him against extraordinary odds, toppling fully-evolved pokemon, pokemon that are immune to electricity and even legendaries like Regice and Latios.

Using Pikachu was not taking a risk in Ash's eyes, rather it was the opposite, a testament to how relable Pikachu was. In this instance I don't see anything wrong with Ash's decision. Both Pikachu and Eevee were first stage evolution Pokemon and Eevee probably was a Pokemon that Gary had acquired recently, not likely to be at a higher level than Pikachu.

Eevee is a rare Pokemon, Gary probably did not find it right away. We know for a fact that it wasn't Gary's starter Pokemon, that was Squirtle. Gary also unlike Ash almost always uses a fully-evolved team, and his Eevee follows this pattern. It was fully-evolved into an Umbreon by the time of Ash's next encounter with it in Johto. The fact that Eevee was not evolved at this time makes me suspect that it probably had not seen many battles yet.

Pikachu should have been a good match. There was nothing I can see that Ash or a Pikachu did wrong. I remember being surprised that Ash didn't win when I saw this episode and feeling bad for Ash. It just appears that Gary had a better strategy on that occasion and outmanuevered him. This can happen to even the best of trainers.

Gary would've just used Blastoise if Ash used Charizard.

Becuase Ash is not as stupid as Red to use a fire pokemon againts a water pokemon.....wait

Update

Unless Pokemon yellow is canon to the anime, and Gary's starter was actually an Eevee

Honestly, I was always under the impression that Ash just trusted Pikachu more. Charizard was iffy for a while, while Pikachu has trusted Ash from nearly the beginning, and Pikachu did always manage to finagle his way around battles and beat opponents that he probably should never be able to beat. So I think that was Ash's train of thought.

I always thought it was silly, though, like boi, this is a really important battle, you might wanna use... not... Pikachu.. but it's Ash.