Many Pokémon fans who were young children in the 1990s grew up with the memory of Pikachu having a black tail. Here are a few explanations that could apply.
Aside from proving Lt. Surge wrong and increasing marketability, there's another overlooked reason why Pikachu wasn't designed to evolve even after losing to Raichu.
Although finishing the Pokédex is one of the games' main objectives, Goh was the first character in the anime to have that objective. This is why.