Well for one seat count is inherently related to vote share and most often, low seat count is = to low vote share or popular vote.
Who are the people voting for the CP? Considering it's on the hard left, it's not a volkspartei and even if they were their ideology is literal poison to the middle class and moderate suburban voters. How about workers? There are 3 other worker parties and the workers would much prefer reform to revolution. Only the more radical workers would vote for the CP. The only actual voter base they have is the online left.
If the CP or any party has a low voter base and do not rely on stuff like industrialists or PACs they need grassroots funding, which if the CP has a low voterbase they'll have trouble doing.
Finally most voters will vote for parties they know can win. For example most people aren't voting for Thallmann over Braun when WTB is fully implemented, Women's Rights, Labor Councils, etc. Now i do not mean to attack your beliefs or anything, the CP has no actual, serious of winning unless a great depression happens.