Because people like to hand wave away the consistently better portrayal that Kaido and BM have gotten over Shiki ever since the Rocks Pirates were revealed
Oda is just giving them more focus because they´re all more relevant to Luffy, while Shiki has no canon focus
Roger, Garp, Blue hair Xebec=Luffy´s parallels(look at the shonen jump cover)
Kaido/Bm=Luffys enemies
Rayleigh=Luffys sensei
WB=marry sue character Oda likes a lot
Shiki, Silver axe, the pirate island guy have no focus because they have no actual connection to Luffy or the plot,but in the lore we know Shiki seperates himself from the other two, Shiki fought Roger as well as Garp and Sengoku(his marine rival), Oda also namedropped him in the SBS as someone he wanted WB to shout out, and ofc Shiki destroying half of MF and breaking out of Impel down was unprecedented when he did that. Him being weaker than Kaido seems fair, and idk about Big Mom but it´s not literally impossible, but it doesn´t really matter? None of the Yonko have beaten eachother, they just clashed a bit.