By the grace of first past the post, winner takes all. This ancient system prevents people from picking shades of grey parties, since they simply don't exist in any significance. And from the other end it doesn't allow parties to split, since it will mean than the smaller block is immediately equal to zero (zero votes, zero seats). In when parties aren't allowed to split, they trend towards reactionism and radicalism, when radicals can hold the whole party "hostage". Applies to both sides btw.
The standard complaint is the opposite. In a generic first past the post two party system you should end up with two barely distinguishable centrist parties.
But the US system is far from generic. Instead it has several tweaks that make it tend towards extremism. The primary system is probably the biggest factor.
