>>58250010The same reason Mega Galarian Slowbro doesn't exist to our knowledge, we simply haven't yet been given the opportunity to expose one to the Mega Stone in question.
It's possible A-Raichu may also be available in L:Z-A and that they're keeping it a secret, but it could also be possible Mega A-Raichu X and Y debuts in Champions instead, assuming they confirm that Mega Stones are capable of working on regional forms as well.
