I learned recently that we can not derive Halacha from Neviim. Why is this?

It is one of the Thirteen Principles of Faith to believe in the eternal authority of the Torah. Only the Torah, and not the prophets, is the sole divinely inspired authority binding for all times. Upon the completion of the Torah, no new law or commandment can be introduced. It says in Vayikra 27:34 "These are the commandments which God commanded Moshe for Bnei Yisrael on Mount Sinai". Our Rabbis infer from the word "These" that any other laws are excluded.(See Shabbat 104a, Yoma 80a, Megilla 2b, Yerushalmi Megilla 1:5.Also Rambam Yesodei Hatora Chap. 9)