We already have a dead person as a member of the crew. Adding another dead person would be repetitive.
Pedro has other overlaps besides being "dead" like Brook.
-Sword like Zoro and Brook
-Poneglyph hunter like Robin
-Smokes like Sanji and Jinbe
-Hair covers eye like Sanji
-Animal like being like Chopper
Pedro has similarities to other Strawhats yet has his own differences that set him apart such as a prehensile tail, spots, and dynamite. This allows for interactions with those he has similarities with while also being able to stand out.
Caribou will probably ride on the Sunny again but you really think Luffy wants Caribou as a crewmate? I don't see that but I could see Caribou in the Grand Fleet 2nd division under Bartolomeo as someone who worships Luffy.