Why did he say "Shome'a Tefilah"?
Radak #1: There (in Tziyon) You will hear prayer. It says about Galus "Sasam Tefilasi" (Eichah 3:8).
Radak #2: You hear the Tefilah of all flesh, and You do not hear ours?! A mortal king can hear from two or three people; he cannot hear from everyone. You hear everyone's Tefilah at once, and everyone prays his own prayer - You do not hear ours?! "Gam Ki Ez'ak va'Ashave'a Sasam Tefilasi", "Sakosa ve'Anan Lach me'Avor Tefilah" (Eichah 3:8, 44).
Malbim: Most who request from a mortal king, they bring their request to his officers, for not all come in front of the king. You Yourself hear Tefilah!
Why will everyone come to Hashem?
Radak: "Yavo Kol Basar Lehishtachavos Lefanai" (Yeshayah 66:23).
Malbim: They all come to You, for You alone hear their request. I.e. You heard their request for rain, and they fulfilled their vows to You.
