One of Elon's first companies was X.com which competed with another company called PayPal. PayPal then ended up buying X.com, and Elon became one of the biggest PayPal shareholders. PayPal was then bought by eBay making Elon even richer and when he cashed out at some point, he took the X.com domain with him. He uses the X.com domain probably because it's brought him a lot of success.