If we talk about Ash’s Pikachu, the reason is pretty simple, Ash’s Pikachu is the Pokemon anime mascot and if Ash’s Pikachu evolve then the pokemon franchise will lost its identity.

The personality they set up for Ash’s Pikachu cannot be replaced by any other Pokemon, and if its evolve it to Raichu it will make it almost unrecognizable from its former self.

This could possibly cause the ratings of the show to plummet. It’s a risk the writers just aren’t willing to take.

Hence they play it safe by leaving Pikachu and it’s “Pika piiiii”s alone.

Then there is a another reason for this which is mention in the pokemon episode its self.

As shown in The Electric Shock ShowdownAsh’s Pikachu is proud to be a Pikachu and can defeat a Raichu without evolving.

It has refused to evolve when Ash asked it to, saying that it would defeat Raichu as a Pikachu.

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However there are two mistakes in the episode

  • Pikachu whacked the Thunder Stone from Ash’s hand with its tail, but in later episodes, including “Climbing the Tower of Success!”, it is shown that if Pikachu even touches the Thunderstone at all, it will evolve instantly. Thus, it should have evolved as soon as it touched the stone.
  • In the “Official Pokemon Handbook”, it says that Raichu is so powerful that it uses its tail for ground but when Raichu came out of its Poké Ball, it didn’t use the tail for the ground.

And then other reason is Ash hasn’t evolved a single Pokemon using an evolution stone over twenty years. He doesn’t like forcing his Pokemon to evolve.

He belives that its a pokemon choice to evolve or not to evolve so he never use the evolution stones.

What are evolution stones ? ( Pikachu Needs Thunder Stone to Evolve )

Evolution Stones (commonly known as Elemental Stones) are crystals with the power of a certain element that instantly evolve certain Pokémon they are used on. There are currently nine different stones for evolving Pokémon.

evolution stones
  • Fire, Water, and Thunder Stones evolve Pokémon of the corresponding Pokémon types, for example Vulpix, Staryu, and Pikachu, respectively.
  • The Moon Stone evolves fairy-like Pokémon, like Clefairy, Nidorino, and Nidorina.
  • Leaf Stones evolve Grass-types, such as Exeggcute and Weepinbell.
  • The Sun Stone, introduced in Pokémon Gold and Silver, evolves the plant Pokémon Sunkern, Petilil and Gloom.
  • Pokémon Diamond and Pearl added Shiny, Dusk, and Dawn Stones:
    • Shiny Stones are used to evolve Togetic and Roselia.
    • Dusk Stones are used to evolve Murkrow and Misdreavus.
    • Dawn Stones are used to evolve female Snorunt and male Kirlia.

There are a handful of other stones related to evolution, although they are not considered Evolution Stones:

  • The Everstone prevents the Pokémon that holds it from evolving.
  • The Sun Shard and Moon Shard (found in Pokémon XD only) simulate the effects of Day and Night, which allows a happy Eevee to evolve into Espeon or Umbreon.
  • When a Slowpoke or Poliwhirl holds the King’s Rock and is traded, it will evolve into Slowking or Politoed, respectively.
  • When a Happiny holds the Oval Stone and levels up, it will evolve into Chansey.

Lets get back to topic then there is this last reason which i mention in the starting of this article and that is the Pokemon Franchise fears that if Pikachu evolves into a Raichu, it will drastically decrease the number of viewers because Pikachu is one of the favourite Pokemon of many viewers.

And it has reason to be there. It has defeated Team Rocket thousands of times. It has defeated Ground-type Pokemon (yep, I know that’s an Anime Error, but still…) It has defeated legendary Pokemon. (Remember Regice and Latios?)

If you’re talking about the one you caught in a game, it’s probably because you don’t have a thunder stone.

he can’t evolve unless it is given a thunder stone. Ash actually has the stone with him, but there are some possible reasons that he is not evolving.

So, it makes sense.