No one knows for sure why BTA's split 'naturally' anymore than we can predict if clowns will host in them. I have one GBTA that has split once in about 3 years (induced) and a RBTA that split (Same tank) after about 3 weeks of it being in my tank for no apparent reason. Myself and others have successfully induced splitting (Or you can venture to frag a nem), but if your question is strictly focused on why it has not split on its own, the answer is, no one really knows. However, in my opinion, our role as keepers is to provide the best possible captive conditions and enjoy the animals we have chosen to place in our tanks...Let nature take its course form there. Best of luck!
To fail to plan is to plan to fail.