Because at its core, as a matter of principle
People don't live in principle, we live in reality, and reality is messy.
Taking into account the full context and effect of actions is important. And in many cases, things which violate our principles can still be, while not ideal, a net positive.
In a world that is still contending with discrimination, not to mention the long-tail effects of historical discrimination, sometimes we are going to see things like minority groups creating spaces by excluding majority group members. And it's just not the same in practice, even if "in principle" it's bad because it's discrimination. Dogmatically sticking to the principle with no regard to the lives of the people living under it only perpetuates the effects of that historical injustice.