The issue the left has with free-speech boards is they more-or-less lean on verifiable facts to cast their arguments, as opposed to arguments being emotionally charged or purely ideological. Free speech boards filter out leftist arguments because they don't hold ground and so they are met with ridicule. This gives the lefty a choice: either back-down and accept defeat (enough of this will slowly convert them) or they can leave and never come back, sticking to their denial/safe space and lowering the numbers of lefty loonies on the site. This is why there is so much media bias online towards free speech boards; because they cannot control the truth. This is also why they fear an unbiased Twitter so much, it is the biggest social media platform on the internet with the widest reach, and if it were to become "unfiltered" it would inevitably become right wing.