Why are Mega Stones and the Pokemon that use them exclusive to different games?

Question

I just noticed that Houndour/oom and Aron/Lairon/Aggron are exclusive to X, but the stones are in Y. What's up with that?

Answer 1

Only GameFreak can give is the exact answer to this question but it's most likelely to promote trading and encourage people to buy both the games.

Comments

Go figure.

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Ya need em for ya pokeydex 2 brah.