Luffy, Law and Kid have finally defeated both Big Mom and Kaido in recent chapters of One Piece, ending the Animal Kingdom Pirates' tyrannical rule over Wano. While they are celebrating their hard-earned victory, news of their new bounties came out. All three of them have received the same bounty, yet their status remains different.
Luffy was already acknowledged as the Fifth Emperor even before coming to Wano, so his being part of the Four Emperors did not come as a surprise. With Big Mom and Kaido down, another slot for a new Emperor opened. But instead of Law or Kid being given that status, it was Buggy who surprisingly snagged the spot -- and there might be a few good reasons why the two young captains were left out.
Unlike Luffy, Law and Kid are yet to stir any notable events in One Piece. Law may have become a Warlord by presenting a hundred hearts to the Marines, but his tenure was uneventful. The only achievement he accomplished during his time as a Warlord was to establish an alliance with Luffy. After that, he somehow turned into a supporting character.
Law's only contribution was providing the Straw Hat Pirates information and impeccable strategies, which Luffy loved not to follow. Even during the fight against Dofflamingo, it was Luffy and his crew who fought the most. Law only truly shined during the fight with Big Mom -- and even then, he and Kid had to work together to take down the Emperor.
Kid is even worse than Law. When Law and Luffy established their alliance, Kid also allied himself with Apoo and Hawkins. However, his supposed new allies turned coat and handed him over to Kaido. Readers of the One Piece manga only saw him again s a prisoner in Udon, where he and Luffy worked as miners while cuffed in Seastone shackles. Similar to Law, his biggest accomplishment was defeating Big Mom. Meanwhile, Luffy was constantly making it into newspaper headlines after arriving in New World.
Upon arriving at New World, Punk Hazard was the Straw Hat Pirates' first stop; this was where the Heart-Straw Hat alliance was established. They then proceeded to Dressrosa, where Luffy once again defeated a Warlord and received his Grand Fleet. When Sanji was abducted by the Big Mom Pirates, half of the Straw Hats followed suit, wreaked havoc on Whole Cake Island and escaped alive. In the recently finished wide-scale battle against the Animal Kingdom Pirates, Luffy was able to defeat Kaido in a one-on-one fight. Comparing his feats and strength to Kid and Law, the latter two simply pale in comparison.
Both Law and Kid also lack manpower and influence. While Luffy already has his Grand Fleet, Law and Kid only have their small crews -- and Law's motley pirate crew can't really be counted on during a big battle. Law and Kid may not be at the level of the Four Emperors yet but both have own merits -- and there is a good chance they will remain Luffy's allies during One Piece's final saga.