The ticker symbols assigned to Futures contracts are broken down into the underlying contract, the month of expiry, and the year of expiry. For example, a crude oil future that expires in January 2015 would have the symbol: CLF15 (CL January 2015).
Here is the code assigned for each month:
|January = F||February = G||March = H||April = J|
|May = K||June = M||July = N||August = Q|
|September = U||October = V||November = X||December = Z|
Looking at the above, it begs the question why some letters are skipped (A, B, etc.).
Due to the long history of the futures market and some unclear historical records, and therefore a few different answers.
One of the most logical answers to the question is that when months were being assigned they chose to use letters that weren't assigned to other futures related items including A = Ask and B = Bid. However, there is no clear answer to why.