Why does it have to be either/or?
For folk on the left, the root of all oppression is class, using the white, working class male as their model (e.g. statements about the "working class" supporting Brexit and Trump.)
So, when women and men of colour, or women generally point out this ignores the systemic racism and sexism they face, it's "splained" they're mistaken. It really
is about class and once the class system is smashed, racism, sexism and other oppression will vanish, too.
If they then suggest the movement do more to tackle sexism, racism, etc. in society or (dare they say it) the movement itself, they can expect to be ignored, told they're being divisive with their "identity politics," shunned and/or monstered. Think back, for example, to the SWP response to the
accusation of rape against a senior member.
Expecting solidarity from folks who know inevitably they'll get thrown under the bus if/when it comes down to the "wider interests of the movement" is just shitty.
The alternative is a movement based on understanding of systemic oppression and privilege as intersectional.