Sure, many people are already afraid of clowns, but the worst thing about Pennywise the Clown from IT is the fact that he's not limited to that smiling, demonic form. Seemingly unkillable, he can become whatever you fear most, and for the original Losers Club from Stephen King's novel, that meant classic Universal Movie Monsters. So why didn't the new IT film go down that road and use horror icons from the 1980s? According to IT director Andy Muschietti, bringing someone like Freddy Krueger into the mix simply wasn't right for the story he was telling. The director elaborated:
Obviously we considered that for a bit, but I wasn't too interested in bringing Freddy Krueger into the mix. I love the story and I love how Stephen King basically makes a portrait of childhood in the '50s. He's very genuine when he brings all the Universal Monsters to the repertoire of incarnation because that's what kids were afraid of. It would be a natural path to try to recreate that in the '80s, but I really wasn't too crazy about bringing stuff like Freddy Krueger into the story. I thought it was a bit too meta with New Line involved in the film. It's distracting and it didn't feel right, for some reason.
Bringing someone like Freddy Krueger into the equation technically would've been more accurate to the spirit of the source material in the sense that The Losers Club is haunted by classic Universal movie monsters in the original novel. In the book, Pennywise takes the form of creatures like The Mummy, Dracula and the Creature from the Black Lagoon to get under their skin. However, bringing the action to the 1980s would've required Andy Muschietti to use iconic slasher monsters like A Nightmare On Elm Street's Freddy Krueger, Friday the 13th's Jason Voorhees, or Halloween's Michael Myers, which would've ultimately felt a bit too on the nose. Stephen King used his set of monsters well in the 1950s world that he initially created, but an IT story in the 1980s required something else.
Elsewhere in his conversation with Ain't It Cool News, Andy Muschietti elaborated further on his decision to move away from pop culture-inspired fears for the members of The Losers Club and focus on phobias that felt "more layered" in nature. Rather than using scary-looking monsters, each scare seems tailored to a deep-seated psychological issue faced by each of the kids -- whether those films are germs, the death of a parent, or the loss of a brother. By pitting them against their fears (rather than pop culture icons), the film can better stick to its coming-of-age themes and give the heroes better resolutions to their respective arcs.
That's not to say that classic 1980s movie monsters don't get a brief shout out in the film. In fact, the marquee of the Derry movie theater features a direct reference to A Nightmare on Elm Street at one point -- possibly even echoing Andy Muschietti's initial thought to include the burned Mr. Krueger in his version of the tale. In the end, the pop culture references were used more as set dressing than key plot points, and that decision arguably gave IT a sense of timelessness.