they didn't change the name because people already knew it by "180SX" so often that they didn't want to up and change the name and confuse people. don't ask me. all Silvia's/180SX's sold in the UK and NZ were badged as 200SX, and the S12 Silvia was badged as 200SX over here, even the later ones with the VG30ET in them.
as for the name changing, i plan to remove all 240SX badging from my car for the clean look. i might replace the stickers in the side windows with "180SX" ones, and i'm desperatley trying to find an ultra-rare "Sileighty" centre panel for the Kouki 180SX tail-lights. these panels are super rare and were put on the limited-run factory Sileighties in Japan.