Because it was easier to reuse that model.Has "K'tinga" ever been said on-screen? Because "D7" was first used in Voyager, referring to the exact same CG model reused for Enterprise.
And no, the name comes from the TMP novelization.
It's best to ignore the K't'inga in ENT and VOY as a continuity error, nothing more.
They made an era appropriate design for ENT, the D4, but the producers said no.