I would have said that it was because of the boost to the normal form, but Beedrill, Ampharos, and PIdgeot got +10 boosts to their normal forms in gen 6 as well, and their megas got +100. I have 2 guesses. The first, which I think is less likely, is that if they gave it a little more SpA and Speed, it would be just slightly better than Deoxys-Attack (an uber powerhouse) as a special attacking glass cannon, and with both normal and Mega Alakazam having better bulk, it could be considered too op. But they could have just added the other 10 to HP Atk or Def which would not benefit it, but Alakazam would still get +100 when mega evolving. The second, and better guess, is that Game Freak decided Mega Alakazam's stats, and then forgot about it when adding more SpD to the normal form. Still, they should have reviewed it, because Beedrill, Ampharos, and PIdgeot got normal form +10 boosts in gen 6 too, and also got +100 in mega form. I guess they just forgot about the mega when adding the boost to the normal form, because mega alakazam couldve just been banned to uber if it got slightly higher special attack and speed (184 SpA and 151 Spe would be the best spread for that, being better than Deoxys-A just enough in speed, so a 252+speed mega alakazam would be slightly faster than a 252+speed deoxys-a, and the SpA would also be as high as possible while still being faster than deoxys-a). Still, I don't see the deoxys-a theory as a good enough reason, I only talked about it because, well, I'm a competitive player and was wondering if they did the +90 to not break the game, but again, +10 in HP Atk or Def would not benefit it but it would be +100. I believe that they decided on Mega Alakazam's stats, and then forgot about it while adding the 10 SpDef points. I am disappointed though, as with other megas they reviewed it after adding the +10, why not alakazam? oh well, thanks for taking time out of your day to read this, and I hope this helped!