There isn't sufficient room to create completely separate queues while maintaining adequate room for guests in mobility devices. It would be too tight. It would also potentially create lesser capacity in the stand by queue, leading to potential backlogs into the "lobby" area.
They can, in theory, pull it off at the existing merge point (LL and Standby), where it branches off into the three concourses, however, that would compromise the standby wait time calculation, as well as potentially lengthen the wait time for LL guests beyond what is considered acceptable, as one experience may be more popular than the other.