Because there are so many minor parties - but the US is basically a two party political process.
StefanM, Also, you are trying to equate
Political parties with ideologically -
generally the same - but not always - and as you mentioned that can affect the
Red/Blue state status.
I just don't see it happening. While plenty of people dislike the current system, having enough people agree on a replacement to ratify an amendment seems unlikely.
I still find these countries with coalition governments fascinating, though.