If you look at the List of Genes you will see that each gene has a power, the gene power, and this helps to determine whether a gene will be dominant or recessive. The White Unicorn skin color has a gene power of 90, making it the highest of the possible colors for Unicorns, so if a Unicorn possesses this gene it will always be white in color. As for pattern, the List of Skins shows that you will get a White Unicorn with the combination of White with Piebald, Normal, or Cream.
So, in other words, the chances of getting a White unicorn are extremely high compared to other skins. This has likely just been bad luck on your part.