Twitter, or "X" as it's now supposed to be called, has suffered a "dramatic decrease" in its popularity on app stores and in other measures of online findability. Searching for the old name, still in wide use among confused normies and indifferent users, now brings up ads from competitors and hinky SEO plays for the abandoned brand. Searching for X, if you're even doing it? Good luck!
Eric Seufert: "Twitter built a ubiquitous, household-name brand over the course of nearly 2 decades and then simply abandoned it, leaving it to be exploited by competitors, unopposed, through the mobile platforms' branded search ads."
If you want a simple answer for why Musk did this, one better than "branding obsession with the letter X," it's because changing Twitter's name is the only likely way he ends up described as its founder. He attempted the same thing at PayPal and it reportedly ended his brief stint as that company's CEO.