They are beautiful animals, both of them. But, despite much miseducation and misconception, white lions and tigers are not subspecies of their kinds. Nor are they typically very healthy, or even responsible to breed. They also aren't endangered-- though all subspecies of tigers and some populations of lions are. They are, in fact, simply genetic outliers of already established subspecies.
The white coloration of these animals is caused by a recessive gene mutation* that can only be achieved consistently through certain reproductive scenarios. There is about a 1 in 10,000 chance of this mutation occurring naturally. For lions, a slightly lighter coloration doesn't harm them much-- they are already pretty light in color and live in the open savannah. But for tigers, having stark white against black stripes in the shadows of the jungle is a death sentence. You can't hunt very well if your prey can see you coming a mile away.
Also, white cats stand out more to human hunters, which is always a bad thing.
In fact, in India, ever since the first white tiger was captured by a Maharajah a century ago, it has been tradition to keep some at the Maharajah's summer palace. But the problem with keeping white tigers (and lions) is that the breeding needed to achieve the white coloration is rather drastic. A male first generation white tiger has a litter of cubs with an orange tiger female. These will all be orange. But if the male first generation white tiger has a second litter with one of his own (orange, second generation) daughters, the white coloration can appear in those (third generation) cubs.