Let's go through these facts :
You know train to kolkata is running for more than 30 years from singrauli, the current Shaktipunj used to run b/w SGRL - HWH , later extended to Jabalpur.
Singrauli...
more... was in ER , then it came in ECR. Also the demographics here at singrauli is of based on the states like Bihar, Jharkhand , UP , WB who are serving in coal mines and at NTPC.
Even for the transport we have UPSRTC buses from singrauli (1 in every 30 minutes ) for the varanasi , Gorakhpur, prayagraj, lucknow and kanpur. UPSRTC have 1 depot in Vindhyanagar - NTPC township. Even in COVID-19 we have buses running for Varanasi , Prayagraj and Kanpur.
Direct train which singrauli had for bhopal is weekly Hwh Bpl exp, That is just weekly train started in early 2000's which is started much later , hence connectivity to howrah was much in prior than to bhopal. Also BPL-SGRL bi-weekly started just 2 years ago.
Bhopal connectivity is needed from here being the capital of state , but the importance of howrah no one can deny for this region. Even as of now the connectivity to kolkata is better than bhopal ( taking count of weekly trains for kolkata) and all trains to kolkata have good occupancy.