Actually both Castlevania 2 and Castlevania 3 had credits.
Castlevania 2 had credits in japanese version and in Castlevania 3 names of some developers appear after you finish "second quest" (complete the game for the second time).
Credits for CV2.
Programmed with the following characteristics:
Invincibility: Akamatsu
Permanance: Iwasa
Philosophy: Togakushi
Masterpiece: Kawanishi
Sensitivity: Hatano
Excellence: Terashima
Ambivalence: Kuwahara
Flourish: Higasa
Admiration: Ohyama
Warm-Heart: Matsuoka
Superiority: Murata
Synchronism: Matsubara
Circumstance: Konami
Source (http://www.castlevaniadungeon.net/games/cv2foreign.html)
Credits for Castlevania 3 (in the end of the video) (http://www.youtube.com/watch?v=J2_cSP1Un6o#)
As far as I know, the only early Castlevania games that didn't had credits are Castlevania (NES) and Haunted Castle.