The Straw Hat Pirates were introduced to the Worst Generation informally during the Sabaody Archipelago Arc of One Piece. This group of Supernovas was expected to make it big in the New World, at least in the context of the story; however, according to Eiichiro Oda, the majority of them were expected to fade away quickly. Among these pirates, Eustass Kid and Trafalgar Law appeared to be big enough to stand shoulder to shoulder with Luffy. In Wano, these two were said to be on the same level as Luffy, but there was something off about their dynamic.
Despite their relative strength, their popularity among fans was somewhat imbalanced. Luffy and Law ranked in the top five in the first global popularity poll, but Kid didn't even make it into the top twenty. Naturally, Luffy took first place because he is the main character, but that doesn't explain why Law outperformed Kid by such a large margin. As things stand, Kid's chances of reaching this level of recognition may have been ruined by the Surgeon of Death.
How Did the Law Gain Popularity?
Law's disproportionate popularity is most likely due to the amount of time audiences have spent with him. The Surgeon of Death played an important role in at least two major arcs. First, he assisted the Straw Hats on Punk Hazard in kidnapping Caesar Clown, which was crucial for the Dressrosa Arc that followed. In that arc, Law and Luffy teamed up to take down the former's former boss, Donquixote Doflaming. He also accompanied the Zou Arc. These three arcs alone lasted from 2012 to 2016, giving viewers plenty of time to get to know this Supernova.
Throughout these arcs, Law received all sorts of development. His backstory, personality, and powers were all broadly expanded upon. It was even revealed that he doesn't like bread; that doesn't influence the story, but it goes to show how much everyone got to know about this one Supernova. All of this served to get the audience more familiar with Law and, thus, more endeared to him.
The merchandising reflected all of this attention paid to Law by the story. While there's probably one way or another to get posters or shirts featuring any Worst Generation member. However, at his peak, Law's keychains and figurines were presented alongside those of characters like Luffy, Chopper, and Ace; that's how big he was. His impact on One Piece as a franchise cannot be understated wherever the story goes with him.
Even as his role in the series has leveled out, he is still treated as one of the faces of the Franchise. He got important roles in movies like One Piece: Stampede and One Piece Film: Red; in the former, Kid had to share the spotlight with the rest of the Worst Generation. Law was even featured as a DLC fighter for Jump Force. He may have been meant to be presented as one member of the Worst Generation, but now he receives treatment comparable to important members like Luffy, Zoro, and Blackbeard.
So, Kid, Where Does This Leave You?
Kid, in comparison to Law, has done almost nothing since the time-skip. Kid was only ever set up as someone Luffy would have a significant interaction with in the future during the two major arcs in which Law was developing. Despite being a pivotal figure in the Wano Arc, he never received the same level of development as Law due to the fact that there were too many other characters to focus on. He had his moments, and they earned him fans, but not nearly enough.
To make matters worse, with One Piece nearing its conclusion, Kid may never achieve the level of popularity that Law did at the peak of his career. Even if he receives the necessary training, it will be too late in the series' run to make a difference. Elbaf may have properly developed his character, but what happens after that is anyone's guess.
It's almost as if Law's unexpected popularity has doomed Kid to mediocrity. He may receive numerous keychains or figurines, but he should not be expected to make significant appearances in movies or Shnen Jump crossover games anytime soon. They may be comparable in terms of strength, but in terms of popularity and influence, Law is light years ahead of Kid.