You have to remember that Pennywise (or simply It) existed as a unique monster, however massive, within Derry's level of the Tower. That monster was tied to a specific reality. The force (the outside evil) sprang from the Deadlights, which are outside of everything else. In short, the Deadlights continue to twinkle and pulse in unending madness, but on the level of the Tower in which Derry resides, the unique avatar of that energy is dead. You have to understand that we are talking metaphysics here. What is more we are talking cosmology and "fhysics," i.e. fiction-physics here. I will, for the sake of simplicity, refer to It as Pennywise for the rest of this post to avoid any confusion with my using it as the pronoun versus It the monster.
I want you to think back to the Ritual of Chüd. When Pennywise is taking Bill's essense (soul) toward the Deadlights, it is to cast him forever outside and remove the threat Bill represents. There is, without a doubt, a bit of vindictiveness there too. Pennywise taunts and talks to Bill as they hurtle through the darkness. Pennywise comments that they might as well talk "while they still can" because the closer Pennywise gets to the Deadlights, the more unintelligible it becomes. This is because as Pennywise returns to the source, it is somewhat absorbed back into the whole. Sentience as we know it is transformed into madness. This seems to imply that the sentient thought, motivations, and even character of Pennywise were born (or reflected) by the level of the Tower to which it came. In a way Pennywise does not exist as a unique entity within the Deadlights. Pennywise only exists when that energy manifested itself on that level of the Tower, i.e. in Derry. It would appear that the great barrier that holds the Deadlights outside has small holes through which the Deadlights pour out. That light doesn't belong. It has no form of its own. It has no real logic of its own. It is madness. It is wrong. It is outside evil. What does light do? It reflects off things. Pennywise, at least as we understand him, was born of two titanic forces, the Deadlights and the Tower upon which that light fell.
This theory, and as it is my own conjecture I can call it nothing else, seems to work with the cosmology that is revealed directly within the book itself. We get a brief glimpse into the mind of Pennywise. This cosmic force of unmaking is enormous beyond the scope of human imagination. Pennywise believes itself to be one of only two cosmic beings (itself and the Turtle). Sometimes Pennywise suspects (and both enraged and frightened by the thought) that there might be another being, i.e. the Other. In short, if my theory is true, some of the Deadlights pass the barrier and hurtle toward a level of the Tower (reality) and leaves the absolute insanity behind. It perceives itself in relation to everything else. It gains a personality of a sort, however self centered and evil. This creature "thinks therefore it is" as opposed to simply being a cosmic amoeba of evil outside energy without a driving mind behind it. This new creature evolves and is further defined by its stay in Derry. It takes on constant forms, not having any of its own, from the minds and notions of people it feeds upon. While it is somewhat horrific to consider, the full shape and style of Pennywise's evil comes from us. It eats people because we think it is supposed to do so. The nature of its cruelty is a reflection of our own. It is the ultimate outsider and yet the application of its evil is merely a mimicry of our own. It cannot create, only copy. Mostly it just destroys because it is easier to destroy than create. Most frightening of all, Pennywise by its long attendance in reality, is taking on the slow but sure ability to create. It has taken countless centuries but it has finally come due. Pennywise will become both mother and father to a brood. This too, I would argue, it got from us. Light reflects and as we reproduce and grow, Pennywise has through its ability to mimic managed the feat as well.
From a metaphysical point of view, the Losers sealed that specific breach from the Deadlights to their level of the Tower when they killed Pennywise. That isn't to say that living, outside evil won't ooze in again, but that unique, specific manifestation is forever lost. This is because it was like any of us, created and colored by reality in which it came into being and the time it spent there. Pennywise didn't exist as Pennywise in the Deadlights and thus Pennywise can't just be replicated by the Deadlights again. The Deadlights needs us to build the perfect beast. Times change and should that light fall upon us again, it a very different monster will arise.
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