Because it only considers fascism from an aesthetic and cultural angle without any regards to the material basis of it and the conditions that fascism arises from.

It's a hazy definition that describes the psychology of fascism more than it describes the phenomenon of fascism itself, and I think—like is the case a most pseudo-radical cultural critique—its analysis can be, and has been, misapplied because there's no solid definition underpinning it.

It's a bit like how if you ask a SocDem for a definition of socialism they'll tell you that it's welfare programs and democracy and restricting corporations and anti-authoritarianism etc.; they'll give you a laundry list of characteristics which fails to form a cohesive analysis that strictly defines their concept, thus leading to them to miss the fact that Bernie was not campaigning on a socialist platform or that AOC/the Nordic countries etc. aren't socialist, and if you challenge them on these matters they'll deny your rebuttal outright because these things just feel socialist to them.

I guess in short, it's a question of vibes vs material analysis.

It's ultimately fairly vague and more of a "vibes check" of fascism than a concrete understanding of how it festers in a country. It doesn't let us stop fascism, it just gives libs ammo to say "(insert enemy country here) ticks 7 of 10 boxes, so they're 70% fascist!"

It is a good starting point to explain to people that fascism does have things you can look out for, but it really shouldn't be someone's only resource for understanding Fascism.