J Street was likely omitted simply because the letters I and J were often indistinguishable from each other (especially when handwritten), and in 18th century English they were still largely interchangeable. (The 1740 “New General English Dictionary” published in London had a single section for I and J, and the standard identification Thomas Jefferson used on his personal possessions was “T.I.”) Having both an “I” and a “J” street would have been redundant at best and confusing at worst, so “J” ended up as the odd man out.