Valid question. I guess it is that time when there are already many other specials running hence pit line not available for SM ( secondary maintenance). Some of the other spl terminating at ERS currently are HTE-ERS, ERS-TBM, Velankanni ERS, Secunderabad ERS, ERS-MAS. In between they also had SBC-ERS which was there for couple of trips.
Ideally DHN-ERS has a very good slot which could have been run regularly. This could have reduced load from other trains from Jharkhand, Odisha