99% of Blues fans speak English as a first language and can identify with Schenn's roots from a working class family. Our broadcast crew and the entire Canadian hockey media has built up his brand as a 'work boots' guy and Schenn is very good with the media. It is very, very easy to picture him as the face of a midwestern franchise. Beyond the brand, he was here for the Cup win and is locked in to a very long term deal.
Buch is my current favorite player on the team, but I know significantly less about him than I do Schenn. I can't relate to his childhood in Russia in remotely the same way I can relate to Schenn's and the language barrier is a real thing (even though I very much enjoy when Buch does talk to the media). He's been with the franchise for far less time than Schenn and he's a UFA in a couple years.
I think Buch is the best player on the team, but the C (especially the public's perception of wearing the C) is about much more than who the best player is. Schenn is the prototype of what most St. Louis people want an athlete to be and that does matter. I don't think anyone here has any idea who the better leader is behind closed doors. We're projecting/speculating based on public optics and those favor Schenn.