Question

Why do the electron shells begin being named with K,L,M,N and not with A,B,C ?

Answer 1

there was a person  named Charles G. Barkla who named the shell as k,l,m,n 
this was because He noticed that atoms appeared to emit two types of X-rays. The two types of X-rays differed in energy and Barkla originally called the higher energy X-ray type A and the lower energy X-ray type B. He later renamed these two types K and L. he then came to know that k type x ray energy and also produced by inner most shell therefore he named the shells k,l,m,n .

Answer 2

they keep k,l,m,n, because that are scientists names