The -100 series designator has been unfashionable for many years now (it was missing from the 777 too).
More recently, it's been almost compulsory to start series designations for new types with 8 (or 800) and higher (Airbus started it with the A380).
An added complication was Boeing's decision in recent years to stop incorporating customer codes in their series designations, so they really only need one or two digits (saves ink).