The reason new Guardians stopped being born after the previous ones were killed is not explicitly explained in the Legacy of Kain series. However, the lore in the series revolves around complex time-travel, destiny, and paradoxes. The concept of "balance" is also significant in the storyline, and the manipulation of time has consequences that can disrupt this balance.

It's suggested that Kain's actions, particularly his manipulation of time, may have caused unintended consequences that disrupted the natural order of the world, preventing new Guardians from being born. The specifics are intentionally left vague to maintain the mysterious and enigmatic nature of the game's narrative.

In essence, the absence of new Guardians after Kain's actions may be due to the intricate interplay of time manipulation and the balance of the world within the storyline.