Radak (2): Nasan saw that Hashem is with David, and he is a proper king, and he will build the Beis ha'Mikdash. A Navi knows only what Hashem tells him. Even Shmuel, who was greater than Nasan, and it says about him "Kol Asher Yedaber Bo Yavo", "v'Lo Hipil mi'Kol Devarav Artzah" (Shmuel I, 9:6, 3:19), he did not know which of Yishai's sons will be king, and thought that it will be Eli'av. All the more so Nasan could err [about who will build the Beis ha'Mikdash]!

Malbim: He answered based on reasoning. Great Tzadikim [like David] may rely on their hearts' decision, especially since "Hashem is with you." Resha'im may not do so, for the Yetzer ha'Ra guides them, and even not Beinonim, for the Yetzer ha'Ra and Yetzer Tov guide them.