It's explained ingame that the Otherworlder was converted into a female with the summoning ritual so it could manifest with it's brood because... they just thought it'd work. The thing about this kind of horror is even the cultists have no idea what are they really doing, and most of the madness originates from the humans warping things either in an effort to bring the incomprehensible into reality through trial and error, just by the sheer lunacy of the way people bend their religions , or just because of pure malice towards the world.
Lovecraft wrote stories about things unknown to him and his sheer terror from things he doesn't understand because they are too new or he read something out of context. Even his extreme racism came from irrational fear of cultures he had no proper knowledge of.
Also it appears it's a different Otherworlder that is actively invading reality through the corruption of the entire ecosystem and manifesting it's harbingers though the fetuses of pregnant women seemingly hijacking the cycle of rebirth, and the cultists just copied it's mechanics to bring forth the one they worshiped. It's also the logical conclusion simply from the fact the Otherworlder represented by the snails IS coming by itself with no help from the cult.
Rabiane presumably succeeded in the first game. The entity with the big eyeball seemingly left on it's own accord after it's attempt to make Rabiane the pathway into the world failed and no other female left alive in the region since the cult didn't seem to have known they'd need women so they were all sacrificed, killed, transformed in mad experiments or killed by the mutated environment.
The second game further confirms the fact shown in the first about the Otherworlders needing one way or another consenting subjects to be able to get birthed into the world. The cult used brainwashing and hypnosis to do so, the snail entity just does it sneakily with hijacking the natural process of birth.
Another horror element is the goddess the nuns worship is a deity of fertility and motherhood, meaning not only the nuns are inherently vulnerable to it's corruption but eventually their goddess might end up preyed upon by this snail god.
Spooky.
