Because he didn’t know any better.
I’ve read quite a few articles about Kane, as well as numerous interviews with Orson, and my view is that he was able to make Citizen Kane the masterpiece it was (and is) because he hadn’t yet learned “the rules” of how to make a conventional film, and so he tried and did things that were “simply not done.”
High ceilings, burying the camera in the floor, special visual effects, intricate sound design, overlapping dialogue, deep focus… he didn’t “stop” himself from doing any of these different and new techniques, as a more experienced director would. And it are these innovations that make Citizen Kane… well, Citizen Kane.
He was also surrounded by “lightning in a bottle” talent, from cinematographer Gregg Toland and co-writer Herman Mankiewicz to composer Bernard Herrmann and actor Joe Cotten… etcetera, etcetera. He had talented people who added tremendous value and collaboration to Welles… and Orson was smart enough to listen and take full advantage of it.
Orson was a very cocksure individual, but he also was very self-aware… He knew what he didn’t know, and so he learned how to make a movie as he was doing it (with great help from Toland). A heck of an education.