The hypothesis is not that simple as genetics is complex working matter as you may all have discovered!
However, in the ever present limited availability of space in captivity and .. yes .. the heavily inbred population of white tigers, it is far more sensible to provide spaces for a conservation priority tiger subspecies than a genetically determined / pre-disposed colour variation in a tiger - which in the wild given its obvious disadvantages in survival would occur only rarely.
Aside, from the space availability and inbreeding arguments, it is the somewhat undesirable effects of over-commercialising a feature in a tiger to try and win over zoo visitors to the importance of conserving and protecting wild tigers.