Why does the heir end up alone in trading legend?

Ill admit I'm not entirely sure why but I do know that heirs that end up alone tend to have more earnings. As well as the fact that there is an attainable 'Beauty' that requires you produce a certain amount of 'End up alone' heirs. You can find out which in the Market in one of the icons toward the top of the screen

( I've acquired all of these particular beauties so the tab is gone for me but you should be able to see it if you have any left to acquire.).