And that last part is the entire point: There simply is no mainstream-relevant occurency of left-wing terrorism and murders any more in the Western world. The most relevant militant group was the German RAF, which has all but dissolved in the early 90s (there are three still on the run and occasionally robbing a bank but that's it). In contrast, murders and other organized violence and terrorism based on right-wing ideology is shockingly commonplace.
With online-based hate speech, it is just the same matter, only a different medium - and there is so much more content that draws in right-wing hate speech (such as anti-immigration stuff, antisemitism, Islamophobia and LGBT hate) than anything that inspires left-wing hate speech. The only thing that comes close to hate speech from the left wing are the "eat the rich" slogan and Stalin/Gulag memes, and that's it.
As a result, it is obvious from the numbers that the right wing will always complain about "we are getting censored and the lefties are not!!!"... well, duh, how about if the complainers would stop doing the things they get moderated for?!