Mature people are more likely to have heard at least a little bit about Bitcoin to know it's probably worth more than $100. And if they didn't know, they'd probably make some level-headed response (e.g. "I'm not sure what Bitcoin is so I'll take $100"), which wouldn't be as entertaining as two ignorant teenage girls.

And as posted above, it might have taken a while for that guy to find anyone choosing $100 over 1bitcoin.