Why is being born all-white a fatal condition to a Thoroughbred foal?

The American Paint horse can be born with Lethal White Syndrome. They have white coats and blue eyes but their colons due not work. As the foals usually die within a couple days, they are euthanized.
https://en.wikipedia.org/wiki/Lethal_white_syndrome

Actually, white thoroughbreds, while rare, are not unknown.

https://www.pinterest.com/pin/78601955975551957/

https://horseracingplanet.com/2020/12/13/japan-idol-horse-sodashi-makes-g1-history/

https://www.youtube.com/watch?v=yy17L3SLsZE

Thoroughbreds can be born white, though it is rare. There are two ways it can be fatal to a Thoroughbred. For one, it may be an indication of a particular genetic disorder, called Lethal White Syndrome, which causes a lack of nerve cells in the colon and leads to death by colic in 72 hours if the foal is not euthanized. LWS is more common in American Paint Horses, but it can appear in Thoroughbreds and other breeds of horse. In other cases, if the Thoroughbred embryo has two dominant white alleles (W/W), it will die in utero because of the prevalence of nonsense mutations of the W allele. (Having said that, there have been white Thoroughbreds who had neither condition and survived to adulthood).

https://en.wikipedia.org/wiki/Dominant_white#Lethality
https://en.wikipedia.org/wiki/Lethal_white_syndrome