In the rest of the Bible these two prophets get a lot of ink. Samuel even gets a couple books named after him. But in 1 Chronicles they play minor supporting roles.
Prophets in ancient Israel were spokesmen (and women) for God and they regularly spoke out against the actions of kings and societies when their deeds weren't in keeping with God's laws. The Bible is filled with stories of prophets who risked their lives and their reputations to deliver God's message. People never want to hear that they're about to have God's wrath come raining down on their heads. Why do you think Jonah ran for it when God told him to go out and prophesy? It's a rough gig.
But in1 Chronicles, Samuel and Nathan are just kind of hanging around agreeing with the king and occasionally passing along some divine wisdom. Samuel delivers the news that David should be king after Saul dies (11:3). And Nathan tells David that his son, Solomon, is going to rule and his house will be blessed forever (17:3-15). But that's about it. No heavy "day of the Lord" pronouncements of doom.
Even though these guys sometimes act as a go-between for David and God, it's clear that Israel's favorite king doesn't really need their help; he can talk to God on his own. He doesn't really need prophets to deliver these messages (though having their approval is a plus). Looks like these prophets might be hunting for new jobs soon.
Samuel and Nathan's stories are much richer and more interesting in the rest of the Bible. The books of Samuel and Kings tell all about Samuel's miraculous origins (it's an awesome story—you should definitely check it out). He led an army against the Philistines. He anointed the first two kings of Israel—Saul and David. And Nathan was the one who warned David how dangerous his adulterous affair with Bathsheba was. He also had a hand in getting David to appoint Solomon to the throne.
Obviously, the folks reading 1 Chronicles would have known all these stories already. They'd been there, done that, and bought the t-shirt. That's probably why the author of Chronicles only uses these prophets when it's absolutely necessary and keeps the spotlight on David. Since this is a book of David-worship, all the Chronicler needs them for is to show that David had their seal of approval.