Is it because it's easier on the singer?
easier to bend?
Maybe. Eb is also a sweet tuning.
I personally love the sound of Eb, D standard and drop D.
well not really if its easier on the singer, but it could match the singers voice better. Though there are many other reasons to why to tune half a step down. Mainly because of the notes you cant play when its tuned in standard.
Vocals and just general betterness. It feels nicer in my opinion, and just has a generally darker sound, I find.