影の王 たそがれ

It got struck by Zekrom lightning and de-powered.

Deus Ex Zekroma's power made it unable for Pikachu to use his electric attacks. Basically, writers used Zekrom as a plot excuse to have Ash lose to a newbie thus starting their "rivalry".

Because God forbid Ash gets the first win in Unova.

Actually, Ash just gave up the battle for Pikachu's sake. He could have beaten it with Iron Tail, but Ash, being the nice and good trainer that he is, stopped the battle and went to the lab to get help.

EDIT: But yeah! WTF WAS UP WITH ZEKROM?!

Zekrom decided that Pikachu had it too good for too long, and struck him with the karma stick. Thus, Pikachu was too weak to battle effectively, and lost.

Or at least that's how I like to see it. ¦3

Arceus asked Zekrom to screw Ash's win over, and Zekrom did so. Then Arceus made Ash dumb enough to only use electric-type attacks and not Iron Tail.

Thus.

Every time Ash goes to a new region, he forgets almost every single thing he knows about Pokémon and his previous journeys, along with the fact that Pikachu is reverted back to LV5.