Here's why:
* "Woke" is the past participle of the verb "to wake".
* "Woken" is used in passive constructions like "I was woken up" or "The alarm has woken me".
* In the sentence "Woken you up", you're using the past participle as a past tense verb, which is incorrect.
The correct way to say it is "woke you up", using the past tense of the verb "to wake".