1st - There's a difference between Underworld Brokers like Doffy and Underworld Emperors like Stussy, Morgans, Peclo, etc. So far, it has been shown to technically mirror some form of the real world. There are producers like CC, middle men like Doffy and buyers like Kaido. Then, there are those who just corner the market on a certain service or product like the Emperors.
2nd - We've gotten to the point where the products from this world have been a minor part of the story to playing major roles in the story. Before timeskip and now has shown a larger leap in importance and understanding of this world. Especially in PH arc.
3rd - There has been many hints that those from the Underworld will return to the story. The silhouettes and the narrative that many make sure not to miss BM's invites. Now, who is brave enough to miss out on this? Before anyone brings up Jigra, he isn't an Emperor. Then there's the Liquor Iron Ore mastermind narrative that was left open by Dragon.
4th - I feel that it's either because they didn't show a battle feat or because many feel it's too late in the game to cater to any other major aspect of the OP world. The Bellamy and BM crew comparisons are duff for collation because these are leaders in their own right for a world that has been hinted where even a non Emperor like Doffy had so much pull. If someone from BM crew goes down, they still have BM. When a Broker like Doffy is taken down, the amount of countries that are affected is ridiculous. So imagine the effect of an Emperor. Morgans can constantly deny the WG, Stussy is a member of CP0, and there are those who can repudiate BM. As we seen with the Heavenly Tribute, the economy matters. Not to say that the SH crew definitely will need this, but as we dig into Kaido and BM's crew, we learn that it heavily mirrors an organization. They have the numbers, information, finance and material to maintain their power. Law pointed out that it's not easy going up against them for some of these same reasons. The Alliance spent the 1st 2 Acts trying to accumulate at least 3 of these.
Unless we want to say that the story can finish without ever mentioning them again, they can't be dismissed like a member of Bellamy's named crew. What weight in the OP world and foreshadowing before their formal introduction was made by Oda? I can't remember anything about Bellamy's crew. I don't see how you can compare Underworld Emperors and members of Bellamy's crew, when the importance in the world is so uneven.