Why are vitamins named after letters?

They were designated with letters according to alphabetical order, the reason of the sudden jump from E to K is that the vitamins F to J where either reclassified, discarded as a vitamin or identified as having a relationship to vitamin B, the same occured with letters M to P and U

Also, vitamin K, being discovered by German scientists and related to coagulation, it was named that way, since it both followed the alphabetical order and had a role in "Koagulation" of the blood after a wound.

Vitamin A, the B vitamins (a whole class), Vitamin C, Vitamin D, Vitamin E and Vitamin K. Other substances had been given letter names at one time, but have since been changed. For instance, Riboflavin was once referred to as Vitamin G, but given its similiarities to other B vitamins, is now referred to as vitamin B2.