I think it's a semi-deliberate pun. It was Planet 10 when people started looking for it. Then it would be "Planet 9", which is confusing. So they called it then

I think it's a semi-deliberate pun. It was Planet 10 when people started looking for it. Then it would be "Planet 9", which is confusing. So they called it then "Planet X" as in 'ex', nerdily harking back to the good old days when Pluto was still a planet.