Well surely he had to keep having possessions, transformations and such. There's only so many games where getting the axe at the end of the bridge or grabbing him by the tail was going to be interesting. They have to keep changing him up.
Yes, also, funnily enough all final phases have something to do with Bowser's stomach/belly, also, there's a weird pattern on the first 3 Paper Mario Final Bosses as well, they followed a certain order being Bowser > Peach > Luigi.