Because a bunch of (not all) people on the left ARE rigid, unforgiving, and dogmatic. It creates an atmosphere of intimidation and chilled speech, especially for people who have problems with being over-scrupulous.
First, they turn a convenient blind eye to dogmaticism on the center and on the right. (And there is plenty!)
Does anyone thing those groups are going to fight to protect trans access to public bathrooms?
No one like that on the left is elected President.
But I also think the movement against the "SJW threat" is a right-wing political program that has much wider aims.
Or am I joining a right-wing bandwagon of fearmongers who are opposed, not to dogmatism and bullying, but to social justice itself?