The actual origin of IT's primary Pennywise the Clown disguise is shrouded in mystery, but a 2017 movie Easter egg teases an interesting clue. To be clear though, we're referring to the origin of the Pennywise form, not the IT creature itself, as the fact that it's an alien monster from another dimension is pretty well established by Stephen King's book. No, the real question is exactly why IT would pick Pennywise to look like most often.
In-universe, IT looking like Pennywise could theoretically be due to a belief that such a form would enable the monster to lure in prey more easily, but considering how often IT drops pretense and just scares the hell out of people, that seems like an odd concern. Outside of universe, King has said he picked a clown guise for his villain precisely because so many people harbor a deep-seated fear of them. It's no wonder too, as clowns' happy painted-on expressions might well be hiding their true motives.
IT Chapter Two directly pointed toward a potential backstory for Pennywise, with the Mrs. Kersh apparition acting like she was Pennywise's daughter. It's unclear if any information IT volunteers can be trusted, but the scenario might well be true. It turns out though that the first IT set this up even further in advance.
IT: How A Movie Easter Egg Teases Pennywise's Origin
One change to the IT movie that caught many fans of Stephen King's book off guard was making Ben the resident Derry historian of the Losers' Club, and not Mike. In the scene that establishes this change, Ben is seen flipping through an old book containing vintage photos of Derry's past. He comes across a still from the Easter egg hunt in 1908 that was turned tragic by an explosion at the nearby Kitchener Ironworks facility, and in that picture, a traveling clown cart can be seen that says Pennywise the Dancing Clown on it. However, the clown depicted on the side only somewhat resembles Pennywise.
Later in the film, when a kidnapped Beverly wakes up in Pennywise's lair, the same clown cart is seen, which is followed by Pennywise's bizarre yet kind of awesome dance sequence. This suggests something IT Chapter Two would also continue to hint it, the idea that Pennywise was in fact a real clown in the early 1900s, IT's titular creature encountered him, and for whatever reason liked the form enough to make a variation of it his default guise. One assumes IT also killed him, because really, why not? That turn of events would also explain the dance, as it was presumably part of the real clown's onstage act.