[adrianmonk]

Also, 401(k) plans were not even possible until a tax law change in 1978 and an IRS ruling in 1981. So there was a lot of time in the pre-401(k) world for things like pensions to evolve. And now that 401(k) plans exist, pensions have gotten way less popular.

I would guess it's mostly old companies who have them. One factor may be that it's tough to transition a company to not having a pension plan once one has been established. At the very least, you create two classes of employees, old employees who have pensions and new employees who don't. It's a form of compensation, so you have to figure out a way to achieve parity in a way that makes everyone feel they're being treated fairly.