I think one factor is the mass media loves to play on this. Their influence in America is aligned more predominantly on the left than the right (YMMV if we were talking about the other developed countries). Far-right is not as common in reality and is easily recognized and the terrorist is able to be stopped. The far-left approach on the other hand is more subtle and nuanced.
This is also what happens when you alienate people. I think a lot of it has to do with division. When you have a large influence misleading part or whole the opinions and ideas of the "other" side because you are ignorant or probably corrupt, anyone on the opposite side get angry or lash out at society. Labeling a whole group of people with the "terrorist" label based on an ideology when some of them are really just silent or non-violent people shows that one has a problem with empathy.
"Far-left" and "Far-right" aren't inherently terrorist acts, they are a set of ideas and values.