It has nothing to do with reddit being 'hardcore-leftist' as you put it, and more to do with the sad state of the US nowadays. Also, try saying anything remotely moderate on any right-leaning sub and see how that works. Really depends on the sub you're browsing.

Also, my anecdotal experience is very different than yours: not a day goes by without me reading some imbecilic comment about american exceptionalism, and where other idiots (I think you'd call them "patriots") gather to upvote/outbid said comment, particularly on large subs.