From a writing perspective probably because Oda didn't want to overload him with titles. Technically speaking, you would probably say that Roger could have been the Pirate King, Most Wanted Man in the World, World's Strongest Man and World's Strongest Swordsman. But that's totally over the top titles for one character to have, even if that character is Roger. There's also the fact that we probably aren't going to see much more of Roger fighting than what we saw the last flashback, Oda not wanting him to be associated with Zoro instead of Luffy, the time of Roger's death meaning it's got little to nothing to do with the present day title and so on.
There might also be some Black Blade shenanigans. You aren't the Pirate King, no matter how strong you are, unless you get to Laugh Tale. Perhaps you aren't the World's Strongest Swordsman either, no matter how strong you are, unless you make a black blade. That is very much looking like a feat only Mihawk and Ryuma have done, so that would make sense to me. Although if Zoro does it by the end of Wano, that theory obviously goes out the window.