Because there is already a difference between being left-wing moderate and right-wing moderate. You are already half a moron if you are a right-wing moderate (provided you don't earn $4 million a month), so you are more likely to be a complete all out moron if you are a right-wing radical. Whereas if you are a left-wing moderate you are a safe and sound person, and you only venture into shallow waters of moronness if you get radical.
Hence, you are more likely to express your views shouting them to the world if you are at complete all out moron level, because once again : les cons ça ose tout, c'est même à ça qu'on les reconnaît.
You really haven't thought this through have you?
Anyway... It can also be a matter of perspective. Many left wing comments that you read may not seem unreasonable or silly to you, since you are likely to, at least partially share the views of the left wing commentators. However, to someone on the right those comments may indeed seem ridiculous.
I was being entirely subjective, yes. I'll maintain nonetheless.
