It's not too expensive for governments to do it, and it doesn't have to be private enterprise. Neither of those things are true. Nor is it even clear that it would be cheaper to contract with some private provider to boost the ISS than for the space agencies to figure it out themselves with the launch vehicles they have. How do you think NASA, ESA, etc build rockets anyway? They contract with Lockheed, Boeing, Airbus, Northrop, and others to build them. Having the whole operation be under the auspices of SpaceX doesn't do much other than make other governments more reluctant to help fund it.