It’s because he bought X.com for the original PayPal back in the 90s and in a roundabout way, it came back to his possession. There are only three single character .com domains registered and he owns one of them. The other 23 letters have been reserved by ICANN.
I think it's because he thinks it's clever, because X marks the spot, or X is a symbol of an unknown like "Solve for X".
And because in old school programming X was a commonly used label for a temporary variable placeholder.
X is also the Roman numeral for 10. So it could be that he figures it as a symbol of a 10 on a 10 out of 10 scale.
But all these are so common and generic it's quite mundane, and you have to be somewhat childish to still think it's cool when you are 50.
But there is of course also the Danish King Christian X, who was king of Denmark during WW2 and the German occupation. And quite the hero here.
But he was strongly against the Nazi's, and protected the Jews in Denmark, so I think it's extremely unlikely to be that.
I always guessed it was cause he was gen x and just ran with it.
But I try not to think about it too much either, cause then I'd be spending thinking time on something wasteful unlike dissecting Simpsons jokes I've heard 50 times, which I find more fulfilling.
That's not as much of sarcasm as some may think, I love trying to find new ways to interpret even the stupidest jokes if there's an angle I haven't considered. There's usually not many but it passes the time as Bowerick Wowbagger the Infinitely Prolonged would say.