1)
Why does it say "ha'Mechaseh Shamayim"?
1.
Radak: Shamayim is high, clear, good, Tahor and radiant. When there are clouds, it is covered from man's eyes.
2.
Malbim: Also in His general conduct to oversee His creations, He covers Shamayim with clouds via Hashgachah, in order to send rain.
2)
Why does it say "ha'Matzmi'ach Harim Chatzir"?
1.
Radak #1: Mountains are dry and hard [yet they are covered with vegetation], and all the more so flat land!
2.
Radak #2: There is no way to irrigate mountains from rivers [and even so, they are covered with vegetation].
