Very few people in the United States have pensions.
Mainly pensions are available for state employees, or union members. And for that matter, state employees tend to be part of a union.
Unions are responsible for pensions, and the only people who have held onto them as the 401k has taken over.
Edit: Because I apparently can't reply to chriseaton below - 401k is defined contribution, you contribute $x which is a set amount and non-taxable. Pension is defined benefit; you receive $x, guaranteed, after x,y,z parameters are met (age, years experience, etc). They are not even slightly the same thing. Not even close.