There's no official reason, however there is a theory:
While the reduction in damage is pretty minor, I imagine the nerf was intended to counterbalance the lack of a special Intimidate a little bit given how prevalent Intimidate tends to be as a check to physical attackers, as well as perhaps to reduce the damage of non-STAB attacks a little bit given how often most of the nerfed attacks are used as coverage moves.
Game Freak seems to be attempting to rework the and balance out the metagame; it's not only Surf, T-Bolt, Flamethrower and Ice Beam that got a power deduction, they raised some of the stats of old Pokemon in order to allow them to function better in the new generation.
You can decide for yourself whether the metagame is less or more broken with the introduction of all these new changes.
Nugget Bridge - Looking ahead: Pokemon XY mechanics