Because everyone knows that the Gs always try to fish out the D. #1 Objective. That's what you're doing.
The Gs know who was investigated (if someone was investigated at all)
If the investigated one was a G, the Gs will prefer the D not to reveal that. Then they will hope to get a lucky whack before he says anything.
If the investigated one was a C, the Gs will try to vote for that person randomly and wait for the D to save him and reveal himself.
That's how you fish for the D. You don't fish for him by telling him to reveal himself.
What I was doing wasn't fishing out for the D. I was actually helping the D how to play this one smartly. To stay quiet if he investigated a C and to reveal himself if he investigated a G because there is no use of that knowledge if he gets killed.
So far the only ones who got random votes were you, me and Janna. In case the D investigated a C, then it's possible that one of the three of us is that C while those who voted for that person are Gs.