This was done really quick since I have to be somewhere in 15 minutes. RUSH RUSH RUSH!!1
Heeere's Asanuma! I hope you've all been reading your manga. -Waggles finger-
Zannid -- Hi! Glad to hear that you're enjoying my thing. Your question is a good one and has a complex answer (I spent some time picking which one to use).
4KH's Zoisite doesn't remember Endymion or Serenity, true, but he doesn't remember Usagi either. So when picking which one to use, he would use the one everyone else remembered - Serenity + Endymion.
Furthermore, Zoisite has some lingering memories of being in the DK and attacking Mamoru + Usagi. Therefore, under his brash and irritating exterior, he's actually very sorry and tries to show respect to them by calling them by their 'royal' names.
And, finally and probably most importantly, Zoisite /desperately wants/ to remember. So he's trying to compensate by calling them Endymion and Serenity - moreso than any of the other Kings, in fact.
tl;dr - BECAUSE
BWAHAHAHAHAHA ALL-BOYS SCHOOL OUT OF NOWHERE!! HILARIOUS TWIST! LOVE that Jadeite's all "fuck this, I'm out"
Mamoru did it on purpose, didn't he?? Oh, this is getting more and more epic every time I read it. You're freaking amazing, Dr. Frizzy! Every new chapter has the same quality of funny, which is, platinum cristal level!