Because as a general rule governments aren't efficient running services and that's fine for a while but long term that means infrastructure isn't properly maintained which leads to service malfunction, which leads to privatization because someone else can do it better and competence appears because someone else things can do it better than the other or to collapse because things don't work anymore.

Source: I am Spanish, a lot of companies were privatized after the dictatorship because they weren't efficient and infrastructure was falling apart. Privatizing allowed competence which meant someone had to invest in infrastructure to be better than the other. Problem is now governments are confabulated with some of those companies to create oligopolies. For example my city has it's own water/trash company (half private half public) so no one is allowed to bid against them; you can imagine how that is going (from paying 35€ every 3 months to paying 65€ in one year), price goes up, no one can't complain or hire a cheaper one, while with cable and phone companies since Telefonica was privatized other companies popped out and you can have cheaper and better internet/phone/tv.