Look around - How many states are the BJP (not NDA) ruling and how may are the Congress (not UPA). Less than the Left.
You are quite right about the CPM not being national in character (they are closer to being traitors than patriots) but please do check the number of individual votes garnered by them (overall and statewise).
Also they are the only idealogues (however pointless) and organised (more than the chadi battalion and monkey dal)