They kept Guinan's nature vague and ambiguous on purpose. I believe the intention was they would fill in the gaps later when the story demanded it. Episodes like Q Who, Yesterday's Enterprise and Time Arrow being heavy Guinan episodes about the nature of her abilities and her origins.
Then came Generations... The moment Braga and Moore had to sincerely commit and define what the rules of Guinan's powers were, and they were underwhelming. Her species is just long lived and they enjoy listening to stories. Hardly the stuff of legends or worth all the mystery that was built up around her character for several years. Like an empty "mystery box", you might say.
If I were to submit an option. Q likely made the same deal with Guinan, that he did with Riker. Granted her the gift of unlimited power and she rejected it. I could see Guinan being unamused at Q's prankster attitude and taunting behavior he plays with lesser beings.