Because Elon is essentially a 13 year old internet edgelord who thinks shit with "X" in it is edgy and cool.
See also: "SpaceX".
I bet he was one of those "Xx420Alphagamer69xX" gamer tags on Xbox live.
Currently playing: Jurassic Word Evo 2, Fortnite STW, WoW, Punch-Out, Super Punch-Out
Convicted criminal Donald Trump: Inmate No. P01135809