Because that's what we were given to choose from.
Insert long, tired diatribe about FPTP voting and the US two party system here.
TL;DR: Third party votes were effectively a vote for Trump. And while I actually did not truly shame anyone for their vote (I hope) this was always true, and I do think folks shouldn't pretend it wasn't true. If you are going to make a principled vote in the name of sending a message, I think it's only reasonable to be honest about the effects of that decision.