The Midrash (Bereishis Rabbah 56:10) tells us that Avraham called the city Yireh and Malki-Tzedek called it Shalem. Hashem combined the two names and called the city Yerushalayim. But we know that Shem preceded Avraham chronologically and he originally named the place. Why did Hashem put Avraham's designation first? Would it have not been more proper to call the city "ShalemYireh"?
The Peirush Yafeh Toar to Bereishis Rabbah 56:10 says that the reason is because Avraham Avinu was a greater Tzadik, so the name he gave (Yireh), takes precedence:
ואף על פי ששם שלם קודם ליראה מכל מקום הקדים בהרכבה יראה דאברהם צדיק תפי משם