If you ask any current-day soldier why there is no J Company, he'll probably tell you it's because in the military phonetic alphabet, J is "Juliet", and no decent soldier would tolerate serving in Juliet Company. But . . . since there was no Juliet-including phonetic alphabet in the 1860's, the most logical reason is that "J" looks too much like "I" to be differentiated in records, so it would be just too confusing to have both. One had to go, and "J" lost out.