The most obvious reason is they are difficult to breed in captivity. They have very long lifespan, long reproductive cycle, low birth rate, and high infant mort

The most obvious reason is they are difficult to breed in captivity. They have very long lifespan, long reproductive cycle, low birth rate, and high infant mortality and stillbirth (at least in captivity). They also don't require a lot of space just because they are physically large... They require a large amount of space because they are elephants, so making them physically smaller wouldn't be helpful in reducing their space requirements. That's all with ignoring all the impossible logistics, massive capital investment, and animal welfare concerns.