Very good question. I have asked this myself. It all comes back to the IAU upon which the subject gets very touchy and people get upset.
Apparently if a, um, "world", fails some criteria it is no longer a planet, yet if a, um, "world" is a huge ball of gas then it IS still a planet. Apparently the definition of a "planet" is very stretchy and rubbery, then it suddenly breaks. Fascinating.