That's because the airwaves are public, so the FCC could regulate / require equal time in the public interest. A TV or radio station isn't operating entirely on its own dime - it's using a public resource, so it should be done in the public interest.

There was no fairness doctrine applied to, say, newspapers.

Right, I meant they're not bound by the first amendment. There are plenty of other regulations that apply to specific outlets, but they're not rules to protect free speech