The typical reason for having missing letters or numbers in a sequence is to keep consistency across the fleet. In this case, you're on an aircraft with 2-4-2 seating. The airline probably also has aircraft with 3-4-3 seating. Knowing that a seat is B or J tells the staff that is a middle seat.

On many smaller aircraft, particularly those with a 2-2 seating configuration (two seats on each side of the aisle), airlines deliberately skip letters like B and E to maintain a consistent industry standard for seat types. In a standard 3-3 layout, 'A' and 'F' are window seats, 'B' and 'E' are middle seats, and 'C' and 'D' are aisle seats. By skipping 'B' and 'E' on a four-abreast plane, the seats are labeled A-C and D-F, which immediately tells a passenger that 'A' is a window and 'C' is an aisle. This logical mapping helps frequent flyers and crew identify seat locations across different aircraft types without having to memorize specific layouts for every plane in the fleet. Additionally, letters like 'I', 'O', and 'Q' are almost universally skipped across all airlines because they are too easily confused with the numbers 1 and 0, which could lead to significant boarding delays and seating disputes among passengers.