[Gurthang]

Maybe it has to do with the translation from Entish. Perhaps in Old Entish, they called them Ent-women, but it changed to Entwife when going to elvish.

Yet here's another thought. The Entwives seemed to stay out of the forests, generally so they could tend their gardens. Perhaps the only time the Ents cared about these Ent-women, was when they became in love and got married. In which case, it would be proper for the Ents to refer to only their wives as Entwives. Although that term would be excluding all other female Ents who had not become married.