I'm sorry, but they are. Sonic is literally EVERYWHERE. Mario is not on TV, not in movies, not in comics, and not a whole lot of shelf space at Toys R Us and Target. Sonic can be found in those places and more.
Remember I'm talking about kids with no sense of brand loyalty.
EDIT: Sonic also has 3 mobile games which have done very well, while Mario has none. Infinitely more exposure than Mario at the moment. I'm not happy about it, and I think Nintendo needs to bite the bullet sooner rather than later and put SOMETHING with Mario on iOS and Android. But how can anybody argue that for as heavily marketed as Mario is, there's nothing to it's brand besides games exclusive to Nintendo systems? The only reason you see the few iterations of Mario amiibo everywhere is because those are the only ones left on the shelf. Otherwise, there's, what, DS plates and accessories? A couple of weird miniature figures here and there that nobody wants? I'm being slightly facetious because I have seen other Mario toys around, but it's shockingly little compared to how big the brand could - and should - be.