Why would you guys wanted a MegaPikachu only for the sake of Ash to look stronger? Isn't Raichu already something similar to Mega Evolved Pikachu?
Go by game-stat-wise, Pikachu's BST is 300, whereas Raichu is 475. I don't think a MegaPikachu will get a stat-boost of 175 that will surpass its normal evolve form. Besides, Pikachu already has Light Ball as its personal power-up item. Giving it a Mega Evolution is just unnecessary.
And go by anime-wise, If Pikachu gets a Mega Evolution, then Ash will probably battle with Pikachu 90% of the time, where it just means steal the screen time of other Pokemon even further. And that gives a perfect excuse of Ash's Pokemon, including Pikachu, to never grow up, because one MegaPikachu is enough to sweep all the others off. The anime is a show about Pokemon, not just Pikachu. I don't want to see Pikachu all the time, I want to see other Pokemon battles as well.