(click images, via) It's, ostensibly, because Spontex sponges are so rough. And Ernie the hedgehog—the brand's mascot—likes it rough. These images from their Facebook page show (top) little Ern in a leather mask, and a sponge wearing a ballgag (translation according to Vanksen, "Stolen Photos of the Hedgehog and Spontex!"). If you're at all sexually experienced, you can figure out the bottom two images. Here's a spot from 2008 showing Ernie rubbing uglies with a Spontex. So: Sex sells sponges. Add it to the list of unsexy products sold with sex.
Related: unsexy products sold with tits.