REI shows that you could get cursed droplets by killing evry mob, but i get it only by killing wither skeleton

That's a side effect of RER. All mobs do drop the droplets, but only when spawned by cursed earth. Otherwise, as you say, it only gets dropped by wither skeles.

As Katorone said, all the mobs with the Cursed Effect will drop Cursed Droplets, but RER does not show it correctly. It should be reported to them.