Because actually governing isn't a boot on the neck of descent. This is the problem with ML dogma. That can as a rule only achieve power through force and violence, and only maintain it through the same.
When you are a congressperson or the mayor of a large diverse city. You have to represent large diverse populations, ideologies and opinions. You have to compromise to govern. If ideological purity is more important to you than good governance. Stay out of government.
I am cautiously hopeful Mamdani might be able to undo a tiny amount of damage that ML ideology created and wealthy boguoise amplified to smear the left and socialism.