Because it was irrelevant. Oscar Finch had nothing to do with the fax. It was the fact that Staplin was killed between sending the two pages that gave Columbo one more clue to prove it was not suicide.

Why would a man commit suicide between sending two jokes?

call that a lining?

It was relevant for the newspaper page. Staplin's fingerprints were only on one side, and that could be a proof of the fake suicide.

I wondered this too when I watched 'Agenda for Murder'.