"And he saw the place from afar" (Genesis 22:4). At first the place was a deep valley. The Holy One, blessed be He, said, "It is not the way of a king to dwell in a valley, but rather in a high place, lofty and beautiful." So the Holy One, blessed be He, raised it up. Therefore it is called Mount Moriah [Har ha-Moriyah], for out of its awe [yir'ah] it was made a mountain.
Why the Valley Was Raised Into Mount Moriah
Yalkut Shimoni on Torah 100:1
וַיַּרְא אֶת הַמָּקוֹם מֵרָחוֹק מִתְחִלָּה הָיָה הַמָּקוֹם עָמוֹק אָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא, אֵין דֶּרֶךְ הַמֶּלֶךְ לִשְׁכֹּן בָּעֵמֶק, אֶלָּא בְּמָקוֹם גָּבוֹהַ מְעוּלֶה וְיָפֶה, וְהִגְבִּיהוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא, לְכָךְ נִקְרָא הַר הַמּוֹרִיָּה, שֶׁמִּיִּרְאָתוֹ נַעֲשָׂה הַר.