It's true that they look similar. One difference is that while the saltwater crocodile is a solitary animal, the Nile crocodile can have large groups. That could be useful if someone wanted to create a river full of crocs.
Well, I never seen a zoo with a river full of crocs, and dwarf caiman are also there as alternative.
People begging for just another almost copy-crocodile don't see that people that rather see unique animals being added in an African expansion, like they want a river full of crocs, other people want porcupine, generuk, red river hog and other unique animals, you know the types that aren't already in the game as model.
Frontier rather listen to the Leopard and Nile Crocodile people though, those models are very easy to create for quick money.
I'm not a fan of mod-animals, but the likes of Nile Crocodile and other-types of giraffes are an exeption, they can be gained from downloading mods easily, unlike Porcupines and Fossa, who look like bleh when created by modders.