Because people don't think there's any point in voting for minor parties as they can't get a foothold under FPTP. I'm convinced far more people would've voted green if they thought it would convert to MPs.
My system would be sub-regional proportional representation. i.e. Sussex would have proportional representation for its 18 (?) constituencies.
I understand where you are coming from. But the danger of such an approach is we end up with something similar to Italy or Israel, where minority parties are able to leverage their power in a coalition to extract policies and concessions that are extreme, and don't represent the views of the vast majority.