Long time ago the English anime dubbing companies used to dub anime songs. But now they don't dub them. Is there a reason?
My guess is to save themselves money. Dubbing a song in English always seemed to be a rare thing anyway. Anime licensing/dubbing companies here in the US are always trying to skimp as much as they can, to keep themselves in the black.
It takes them extra time and money to find a singer to sing the song in English, get access to the instrumental version of the song (unless they decide to create their own instrumental, which is even more expensive), give the singer time to learn and practice the song, and then get them in to record said song.
The alternative is just leaving the song as it is, in Japanese, and slapping on subtitles for the lyrics.
For these companies, it just isn't worth the hassle. When they do it, it's because they believe the show will sell better this way.