Malbim: (a) An established king must build it. David was shortly after the Shoftim; they were not kings, for their sons did not reign after them. Also Sha'ul's kingship did not last. ¹ (b) Respite from all enemies is required. During the Shoftim, even when the enemy was subdued, they were not eradicated, and after the Shofet died, the enemy attacked again. Also David subdued his enemies, but did not eradicate them. He himself needed to fight more wars (Perek 8)! The Beis ha'Mikdash is for permanent dwelling of the Shechinah - after Yisrael are purified from sin, and will not return to sin after the leader dies.

Malbim (11): This is not explained here, rather, in Divrei ha'Yamim. Nasan did not explain it now; it was via a later Navi, or through Ru'ach ha'Kodesh ² . It is because David fought great wars. He did not have rest until his old age; the Beis ha'Mikdash demonstrates Shalom and rest! Shlomo will have rest from his youth; he will build it.