After 6 months, they were smug and starting to get old. The series was no longer fresh.
That's when Apple and their marketing group decided to run the ads for another few years, until everyone, even people like my parents who don't care about computers, disliked the ads.
No, it's actually because the market for technology has been growing as a whole. Selling more than the previous fiscal year should be a habit, not a great feat for a tech company like Apple or Microsoft.
To be honest those ads didn't improve my faith in Apple or what I think of them - I just found them entertaining. The PC character was good with his dry sense of humor too, although the Mac character was a bit sickly.
What I find good is when companies poke fun at themselves. Fox does that TONS on The Simpsons.