Think this through: Pikachu hates it's Pokeball. And I mean HATES it. A Pokeball is designed to store a Pokemon for easy transportation, both on the part of the Pokemon itself and it's trainer. Because Pikachu is never in it's Pokeball, it always rides on Ash's shoulder. This is feasible due to it's small size and relatively light weight.
When a Pikachu evolves into Raichu, it doubles in height and QUINTUPLES it's weight. Therefore, riding on Ash's shoulder is no longer feasible. Therefore, Pikachu would then either have to get over it's hatred of it's Pokeball (which it doesn't want to do), or walk (which would tire it out and leave it less ready for combat).
It's a lot easier simply from a transportation point of view for it to stay a Pikachu. It's personal preferences in that regard aside.
If Raichu gets a mega evo this gen, that might be the highest possibility of Pikachu ever evolving.
Even then, that's a slim chance.
Actually, now that I think of it, the anime writers could probably just find a way to have Pikachu warp-megavolve into Mega Raichu temporarily when it needs to. That way it can go back to Pikachu >_> --- "I am champion of Bellator. I am face of Bellator now. Who want with belt? Come on with cage. I am beat you. -Alexander Shlemenko