Malbim: Hashem told David that if his acts are pure, without ulterior motives, He will establish Kisei Mamlactecha, i.e. ruling over Yisrael permanently. There was no Bris about this. Here Ezra taught that there was also a Bris that if he guards Hashem's Mitzvos, even Lo Lishmah, He will establish Kisei Meluchah, i.e. ruling over part of Yisrael (not necessarily permanently).