Because it is the truth that Ryuma fought against the world government which didn’t exist during Joyboys era as the WH was established after the void century.
The simple answer to this question is simple; Ryuma survived the void century, hence him being the first daimyo of hakumai after the establishment of the new Wano and starting the Shimotsuki line of Daimyos in the first place.
It would make sense for the world government to target the country of Wano as soon as it’s established for 3 main reasons;
1) The acquisition of the ancient weapon Pluton
2) Wano was originally part of the 20 kingdom so they’d want to overthrow the Kozuki and instal the Kurozumi instead
3) They needed money and therefore the gold as a newly established organization and not many countries have joined thus less heavenly tribute
While Ryuma is a known individual despite being from the void century, him being joy boys right hand man and involvement with void century events are what’s hidden and that’s because he outlived the Void century