Tziklag is listed among Shimon, and above (15:31) amidst Yehudah, i.e. Shimon received it amidst Yehudah's portion. Why was it to kings of Yehudah from the time of David?
Radak (15:32): Yisrael conquered it, and it fell to Shimon. Later, Melech Gas took it from them, until David dwelled there. From then on, it was to Malchei Yehudah. Malbim (6)