That's because the academic definition sets the limit of the term, while the informal understanding connotes the damaging practice.

People on the right do it, too, but worse. Take critical race theorists as the prime example. The term refers to an incredibly small subsection of academics who combine critical theory with critiques about race. And somehow, this minuscule aspect of even legal academia is everywhere all the time.

I personally don't have a problem with connotation/denotation blending. Christian nationalists were studied because some academics found their beliefs interesting enough to study. And it was interesting to study probably because they didn't seen Christian nationalism as extremely benign. They wanted to systematically survey the danger that the belief system represents.

Fascism has an academic definition, too, and it's been studied a lot since its inception. Fascism is a dangerous ideology. A systematic inquiry into how it operates can help those who love freedom push back against it.

The difference between the left and the right isn't whether the blending occurs, but that the right isn't disciplined in its use of the method. You don't see leftists disparaging Trump a Christian nationalist like the right calls regular history teachers critical race theorists. Instead, they leftists rightfully call Trump a fascist, while the right manufactures and exploits racists fears about white subjugation or even extermination in a pluralistic society.