Well, the reason why there is no more LoK games is because Crystal Dynamics became the new Tomb Raider developer soon after LoK: Defiance was released.
Well I think the Original blood omen didn't have what it needed to become a big serie like Tomb Raider, I think it was old school but way too early for old school to be popular (ok you could always argue that Symphony of the night was a popular old school game, but Castlevania already had a fan base unlike LoK)
Still think the ambiguous ending was kinda good and rather fitting considering the overall tone of the game serie (the only point that really need to be explain probably how the hell Vorador manage to be alive in Blood Omens 2 but that can be explain by the shifting in the timeline and crappy edge of Blood Omens 2)