Well, Anthony, that's really the $64,000 question. Why indeed? I offer two short answers:
1. Hagen fell down on the job--again. He should have known about Pentangeli's survival because he had "people with the New York detectives." If you did, Tom, why in hell didn't they tell you about Frankie's survival--and why didn't you check with them before advising your one and only client to perjure himself five times before a Senate committee? DUHH-H, Tom--that's one of the reasons why you're not a wartime consigliere.

2. (Attention, Apple!): It was a dramatic nuance designed to set up two of the Trilogy's absolute best scenes: The confrontation with Michael and Fredo in the boathouse, and Pentangeli's sudden about-face when his brother showed up at the hearing.

I have a longer theory, but I'll wait for others to post.