I think part of the reason for rare mentions of the true composer is that a lot of covers of songs are done by popular artists and perhaps the motivation is to boost up sales for the artist by acting as if the song is original even when if was written by someone else. CD producers or whoever is in charge of writing attributions in liner notes or the people who run lyric sites may also believe that most people wouldn't want to know that information.

Just a thought
Morwen.