The reason for this was a genetic mutation. Having double rows of eyelashes is usually the result of a mutation at the FOXC2 gene, a gene that influences kinds of tissue development in embryos.
Though having extra eyelashes was certainly helpful to Elizabeth Taylor’s appearance and career (as she too had this mutation), it carries another problem.
Lymphedema-distichiasis syndrome, caused by the mutation, is a hereditary disease that can cause disorders of the lymphatic system. This actually contributed to Elizabeth Taylor’s death last year, which was due to heart problems.
The extra eyelashes also aren’t always a good thing. Often, they can grow inward, and if you’ve ever had an ingrown hair, you know one of the last places you’d want one is the eye. They could also grow in the wrong direction and damage the cornea. So with the added risk of heart problems, perhaps the aesthetic value isn’t worth the envy.