Probably because Pawan express was extended beyond Varanasi to go to MFP. Also, LTT-BGP Janata express was short terminated at Patna. So, as a compensation, this train was diverted to BGP. There were probably plans to extend it all the way to GHY ( as it was extended for one day as 15647/48 ) but the plans were probably dropped later on due to the demand from BGP and neighboring regions in Bihar.