Often as a cause of two-party system in the US cited the first-past-the-post systemcause of two-party system in the US cited the first-past-the-post system. (see also Duverger's Law). But theoretically this should not prevent having strong local state-level parties.
It seems that the political life in the US would be richer if it had several tens of locally strong parties, even if only two parties would participate in presidential elections.
Or, maybe, the cause is because the Americans cannot be members of two parties at the same time, so that if you are a member of a local party, you are excluded from Democrat and Republican primaries?