Let us remember what this poem is all about. This poem represents a dive into nostalgia as the feel of the West Wind reminds the poet of home and of all that is good there. The entire poem is an evocation of how wonderful life is like in his home, and the description of the natural elements reinforces this theme. Consider how the rain your question refers to is described:
"Will ye not come home brother? ye have been long away,
It's April, and blossom time, and white is the may;
And bright is the sun brother, and warm is the rain,--
Will ye not come home, brother, home to us again?
This is the voice that the wind carries to the speaker. Of course, this is an exaggeration. Anybody who has been away from home for a long time forgets the realities of what life is like for them back in their home. Speaking as somebody from England myself who has been away from his country for over a year, I find myself having similarly nostalgic thoughts about England, but at the same time I remember that the rain in England is hardly ever "warm" and is actually mostly cold and miserable, but describing the rain in this way would rather detract from the tone of nostalgia!