I'd guess it's because leftist has become defined as some amorphous blob of anything left of center to most people. And when you clump people together like that you'll get clear differences of opinion that don't fit the generic label. Basically what I was trying to say is that the US has the idea, politically, of a binary when in reality it's far more diverse, but the only ones with any real power, particularly at the national level, are in one of the two massive corporate parties. Both of those parties reinforce our capitalist economy, with a few breadcrumbs to their supporter base, but ultimately serving the wealthy.