Five times we got to see those freaky guys in ape suits talking without moving their mouths, hardly. The waning energy of the civil rights movement probably also took some of the steam out of the Ape movies. By 1979, with the first Alien movie, creatures got slimy. All the Aliens are full of slime, bubbling eggs, a shocking variety of pregnancy that can strike any gender, fluid, afterbirth, and vicious maternal love. If this sounds far-fetched, keep in mind that some have theorized that human culture is largely a construction of men trying to compensate for the fact that they are incapable of bringing forth live young.
Birth can be a powerful, disturbing thing.
Interestingly, it's the depth of the fear of reproduction that has allowed the series to reproduce itself for so long. Shortly after they clone her up in the lab, a gooey little piranha is extracted from her stomach via C-section. We already know that these little ladies are bad company. This particular one is a queen, we learn through some clunky expository dialogue. Before we know it, the entire ship has been converted into a monster breeding center.