[ncmncm]

Because the line is very hard to discern, where it exists as a line at all, and it is nowhere sharp. Lions can be bred with tigers, in captivity. Are their offspring fertile? Well, sorta. Does it make sense to call lions and tigers subspecies? Hell, no. Say lions and tigers are one species and biologists will call you a lumper. You don't want that.

Sometimes the product of mating between species becomes, instantly, another species, if they prefer mating with one another over either progenitor. That just happened, with some birds, in the Galapagos.

Legally, there are no endangered subspecies, only endangered species. So, claiming some variety is "just a subspecies" may mean they get no legal protection against extermination. To me that's more than enough reason for a species.