This is definitely an Australian thing too... rival digestive yogurts are duelling through most prime time ad breaks at the moment. Also there's a charming new ad set inside a woman's bowels, involving a home makeover metaphor.
Besides the psychologistic explanations others have jumped in with, I'd suggest its taking off partly because the marketing people have discovered a latent medical anxiety ripe for exploitation - and it's one that can be targeted with dietary supplements rather than highly regulated drugs.
Mike Beggs scandalum.wordpress.com