Perhaps it's because right wing extremists are significantly more likely to use violence? I think it's understandable that the default image of a terrorist is someone who espouses right wing values in that case. https://www.theguardian.com/us-news/2021/sep/08/post-911-dom...

I might be in a bubble but I very, very rarely learn about left wing attacks, while right wing ones pop up here and there.

Edit: Also, about that link GP posted above about the Congressional baseball shooting: The perpetrator attacked Republican lawmakers, who have the most power to implement right-wing policies. In contrast, the attacks mentioned in my link mostly affected random people who probably did not have much political power, and were just members of groups that are often disliked by right wing extremists.