Because he had killed Nob, the city of the priests; and because he had spared Agag; and because he had not listened to Samuel, as it is said, "Seven days you shall wait" etc. (I Samuel 10:8), and he did not do so, but rather, "So I forced myself, and offered the burnt offering" (I Samuel 13:12); and also because he inquired of a ghost, as it is said, "And also for inquiring of a ghost, to seek counsel" (I Chronicles 10:13), "and he did not inquire of the Lord; therefore He put him to death" (ibid., ibid. 14).
Why King Saul Lost the Kingship in Midrash Tanchuma Emor
Midrash Tanchuma Buber, Emor 4:2
על שהרג נוב עיר הכהנים, ועל שחמל על אגג, ועל שלא שמע לשמואל, שנאמר שבעת ימים תוחל וגו' (ש"א י ח), והוא לא עשה כן, אלא ואתאפק ואעלה העולה (ש"א יג יב), וגם אשר שאל באוב, שנאמר וגם לשאול באוב לדרוש, (דה"א י יג), ולא דרש בה' וימיתהו (שם שם יד).