In the book, the whole point of Pennywise in his clown guise was to lure kids in by appearing harmless fun and goofy. The first part of the novel takes place in 1957, decades before the whole ironic creepy clown trope was a thing. Although creepy clowns have always been around, it wasn't as prevelant back then. A clown was still a credible disguise. \nBy moving the setting up 30 years, you make Pennywise's modus operendi rather dubious. Not to mention the fact that Bill Skarsgard didn't even try to appear anything other than a creepy monster.