Rajan,
One of the problems that larger animals face is the need to have a longer organ to reach to and into the female part. With increased length and force behind it, there's danger of tissue damage.
As part of the cooperative nature of the elephant, several others will place themselves either side of the female to be mounted to help steady her and allow the penetration to proceed without damage.
Then, some animals, to protect their speciation, (or allow prolonged mating with rarely discovered females), have a bone to define the geometry. There's immediacy of function. Also there's less chance of crossing by competitors with the wrong key shape. I'd be worried about damage to the lock! More
here.
Asher