Broadly, because Marxism isn't the same thing as Leninism and because non-Leninist Marxism, though it wanted more radical change, was a central source of ideas, organization, and activism in the movement from classical capitalism to the modern mixed economy throughout the developed world.
> it wanted more radical change, was a central source of ideas
You could say the same thing about Fascism
To attempt to dispense with either: Marxism very much was and Fascism very much was not an important source of ideas, activism, and organization driving the transition from relatively pure capitalism (“pure” in the sense of being essentially the dominant system in the developed West criticized under that name by Marx, not pure in the sense of embodying the post hoc idealistic rationalizations invented in response to criticism of that system) to the modern mixed economy.