The film addresses this. The Rebellion did not know how Saw would react as by now the connection between the Rebellion at large and Saw's group is tense to an extreme degree. Saw doesn't trust them, and has become increasingly paranoid. We see this both at how Saw reacts when they bring him Bodhi (emphasis mine):
SAW GERRERA: Every day, more lies.
BODHI: Lies? Would I risk everything for a lie? No, we don’t have time for this! I have to speak to Saw Gerrera before it’s too late.
The sack is pulled off of Bodhi’s head.
BODHI: Okay... So, you... Um. Uh.That’s-That’s for you. And I gave it to them, they did not find it! I gave it to them. Galen Erso. He told me to find you.
SAW GERRERA: Bor Gullet.
BODHI: Bor Gullet? Galen Erso sent me! He told me to find you!
And then there is the conversation he has with Jyn which kind of proves again they had reason to try and find someone Saw would trust (emphasis again mine):
SAW GERRERA: You were the daughter of an Imperial science officer! People were starting to figure that out. People who wanted to- to use you as a hostage. Not a day goes by that I don’t think of you. But today, of all days? It’s a trap, isn’t it?
JYN ERSO: What?
SAW GERRERA: The pilot! The message. All of it. Did they... send you? Did you come here to kill me? There’s not much of me left.
Essentially they knew that Saw Guerrera by this point could react like this, the safest bet was to get someone they hoped he already trusted rather than just any random person sent by the rebellion.