Malbim (15-16): (a) The nation did not cease its abominations. Menasheh told them to serve Hashem, but they did not repent. Hashem also punished them for their fathers' sins, since they continued in them (Brachos 7a) - "Poked Avon Avos Al Banim" (Shemos 20:5). (b) Menasheh repented from idolatry, but not from murder and his other abominations - "v'Gam Dam Naki Shafach." This is why Sefer Melachim does not say that he repented.