Because Sanji is weak. He couldn't even one shotted page one with rs. They were even. Unless king isn't as strong as we expected. Maybe Page One is close with him in strength
Page One's DF is Spinosaurus, King's is a Pterodactyl (as far as I remember). Page One's animal is much more durable in nature. King, due to being YC1, might have more durability (Haki, awakening etc.) and should have overall more fighting prowess, but the difference in durability probably is not that high. (Queen, due to his DF, his YC2 status and his physical build, is probably the most durable BP member)
Luffy couldn't one shot Page One too. My boy got struck with an elephant gun below the chin (one of the least durable parts of the body even if you defend with Haki) and was perfectly fine like what, 2 minutes later? He is extremely durable.
At this stage, DF's content matters too. Instead of looking at it like "both of them are dinosaurs, so stronger person should also be more durable", consider the types of dinosaurs in these types of cases. (Spinosaurus and Brachiosaurus are more durable than Pterodactyl)