those who want to add this title to their names usually want their reward for Hajj from 'awaam and NOT from Allah. those are the ones who go for Hajj empty handed and come back empty handed.
Allah listens to their innate desired to be called Haajii/alHaaj and they reap their rewards from public with no share in the hereafter.
But is it sub-continent specific phenomenon or they got it from Arabs or Persians?
as far as i've seen arabs...they do NOT use this title...i'm pretty sure it's south east Asian phenomenon...subcontinent, Malaysia, Indonesia etc.