They were just alphabetical, A was the first one they found, B the second, etc.

Except, K was named “Koagulation” from the fact it helps blood coagulate

And also, the “B vitamins” were originally one extract, a serum used to treat a disease caused by B vitamin deficiency. Later, we learned that serum actually had several different molecules in it, now seperated as B2, B6, etc.