We also have some fluidity to our parties - right now there's a right-wing, a left-wing, and a central-left-ish party, plus a fringe far left party. These groups sometimes separate or merge - there used to be a far-right party (similar ideologies to the US republicans actually) which consolidated with the more sane right-wingers to form a right wing party that more people felt comfortable voting for. These shifts probably help maintain the viability of the multiple parties.
I'm just theorizing, though. I don't know that much about politics and know even less about US politics. I just know that I see a ton of US people frothing at the mouth about politics (yes not all or even most of you, I know) but almost never hear anything close to that level of vitriol here. I guess there are also minor cultural differences to consider as well. We tend to be less....angry in general, for whatever reason. Still, it seems like people in the US often identify themselves really strongly with their political party, as part of a core identity or something, and here a more common attitude is to pick your party based on your core values, where the platform is more important than the party name. Maybe that's a small distinction but it seems noticeable to me. Maybe if your party is part of your identity, you feel personally attacked when someone disagrees with their platform, or something. I don't know.