R2-D2 was an intelligent droid.
He knew much more than most people expected of him, which generally left him in a position of power that would be disasterous if he decided to leak everything.
For example: Anakin and Padmé. The idiots were married! It's a true miracle that nobody caught them yet. Except, oh wait! Ahsoka.
Ahsoka knew about Anakin and Padmé, just not to the same extent that R2-D2 did.
"Okay," she whispered one day, leaning in close and placing an orange hand on his dome, "I don't know if you know or not, but I saw something, and I have to tell someone. They never swore me to secrecy, exactly-"
R2 beeped out an inpatient response which pretty much translated to "get on with it already."
And get on with it she did.
"They were kissing, Artooie!"
R2 then beeped out the equivalent of "well, yeah. I was there at their wedding. It's about time somebody else found out."
His friend simply sighed and walked away.