Because they're not a threat.
At least, not yet.
Kuzan beat Luffy without even extending an effort. The only reason the Gorosei is concerned is because he beat Crocodile, which could have thrown the balance of power out of effect.
Kuzan doesn't respect Luffy at all. But he's fought with the D's before (Saul, and most likely Roger). He knows that they all have potential to become something. Saul was mighty powerful in his own right. You naysayers that say Tom was stronger are stupid. Tom lifted up REGULAR SIZE Galleons that were made by Franky. Saul lifted up a 5000 TON BATTLESHIP. There's a HUGE difference.
I'm sure if Whitebeard demonstrated his strength, we could see him Smash a mountain in two, practically. But then again, we don't know his fighting style, or Roger's. I'm under the impression Roger fought with a spear, based on a lot of things, including who he was named after (Woodes Rogers), his name itself (Roger, which means Famous Spear in German), and also the scar on Whitebeard's chest. If Whitebeard was the only person to come out even, then that scar wasn't caused by something stupid. It had to be caused by Roger.
Kuzan wants to see if Luffy will become like his grandfather. And I want you to note something, too. Kuzan says to Luffy:
"You're just like your Grandfather, Monkey D. Luffy…"
Notice a key element. There is no WAS in that sentence. Does this mean Saul is still alive?
Let's consider a few things:
1. The move that he used on Saul was called "Ice TIME CAPSULE"
2. When Robin and Luffy were frozen, they were able to be thawed out.
3. Luffy has been impaled, blown up, Set on fire, dehydrated, and he came back from all of that. I seriously doubt a D would go down from two moves (Ice Partisan and Ice Time Capsule).
Do you think all these people are guiding the D's along a certain path?
Let's think about this:
1. Shanks knows Luffy and Ace, and probably the other D's as well. His letter to Whitebeard mentioned two of the D's: Ace and Blackbeard.
2. Whitebeard had two D's in his crew: Ace and Blackbeard. Do you think he knew that both were D's? Probably. The fact that he didn't tell Ace that Blackbeard was a D also is a mystery.
3. Several people have alluded to "The will of the D". Robin, Kureha, Woop Slap, Kokoro, Shanks (vaguely), and Kuzan.
4. Kuzan instantly located the Strawhats. This either means that Robin has a tracker on her, or the D's can somehow be tracked. As far as we know, no marine ships have been encountered since they left Alabasta, so intelligence on their position could be given. Kuzan either knows how a D thinks or knows their location through some sort of positioning device.
Best guess? Kuzan wants all the D's together, and he's orchestrating it. Here's what I can garner:
1. The Buster call fails and Enies Lobby is Destroyed. This news goes out all over the world.
2. The entire Strawhats get an elevated bounty. This catches the eye of Whitebeard & Blackbeard.
3. Whitebeard sends his first Division head to find the Strawhats and bring them to him. He also sends a messenger to find Ace and order him to be recalled.
4. Luffy and crew meet up with the Blackbeard Pirates again. Mugiwara and Blackbeard fight, when the first division head crashes the party. He tells everyone that Whitebeard wants to meet with them all.
5. Messenger finds Ace and tells him to come back. Ace refuses, but then is lied to and told that Blackbeard is already in their custody. Ace reluctantly returns.
6. Whitebeard meets with them all, explains about the D's and their relationship. Explains that in order to find One Piece, they need Saul back.
7. Aokiji, Kizeru, Akainu, Sengoku, and the Schichibukai appear and surround Whitebeard's fleet. Massive battle ensues.
8. Pirates win, but Whitebeard is mortally injured. Before he dies, he tells Luffy and the other D's that the key to reaching Raftel lies with rescuing the Last D.
9. Attack on Marijoi begins.