Question

Which of these best describes why mainstream media might be called more "accountable" or "responsible" for reports?

Censorship prevents access to certain information and keeps the public safe.
Events are covered round the clock.
There are clear-cut procedures for fact-checking and consequences for breaking these rules.

Answer 1

There are clear-cut procedures for fact-checking and consequences for breaking these rules.

This ensures the validity of the facts. If for instance you are reporting on a recent statement a politician made, do not pull this info from a reddit page or something not formal like tumblr.