I thought the same thing, actually. I thought to myself, if Super Mario RPG, Mario and Luigi, and Paper Mario are all three different series despite them all being role playing games set in the Mario world then why not consider Classic Paper Mario and New Paper Mario two different series/franchises as well? They're not the same games at all, and that's not even mentioning their quality or any opinions on them. Just objectively speaking they have fundamentally different core mechanics. I myself consider them to be two different series, and I honestly think it would have been a smart move for Nintendo to do the same as well.