On some planes, the bulkhead wall is a foot or two farther away than the seat in front for other seats. So you'd need a much longer bracket, or else the tray would be too far away.

On other planes, the aisle seats may not have a full bulkhead in front of them, as the gap between the bulkheads may be wider than the coach aisle, or misaligned on 2x3 planes like some MD80s.