reb moshe lived in america before religious jewry was significant in israel, so it could be at the time it was not an option
And Rav Mordechai Gifter, Rav Yaakov Kaminetzky, Rav Avigdor Miller, Rav Avraham Pam, Rav Yitzchok Hutner, Rav Gedalia Schorr, the Bobover Rebbe, the Satmar Rebbe, the Skulener Rebbe, Rav Elya Svei, Rav Joseph B. Soloveitchik, Rav Hershel Schachter and the many more I didn’t mention including many current Gedolei Yisroel shlit”a?