Because if you vote for Party A and x number of people vote for party A, Party A gets x number of seats. You have a parliamentary system. Seats are awarded based on the percentage of votes. You vote for Party B Party A gets fewer seats, you are awarding mediocrity. If Party A and B are actually similar, then they will form a coalition and simply out number Party C making Party C unable to do anything.
You do not have a two party system. You have a Parliamentary system. You do not have a winner take all zero sum system, you have a parliamentary system.