Saw Gerrera had a falling out with the Rebel Alliance over his use of extreme tactics, and would not agree to meet with them. Any agents they sent would likely be killed unless they were someone who already knew Saw personally. When Jyn Erso is first brought before the Alliance leadership they mention this exact problem.
>CASSIAN: We know how to find him, that’s not our problem. What we need is someone who gets us through the door without being killed.

>JYN ERSO: You’re all rebels, aren’t you?

>MON MOTHMA: Yes, but Saw Gerrera’s an extremist. He’s been fighting on his own since he broke with the Rebellion. His militancy has caused the Alliance a great many problems. We have no choice now but to try to mend that broken trust.