Reasons are as following:
1. SRC-ANVT exp already running via that route & being very longer one not popular among pax.
2. It is difficult to start from Howrah, Sealdah which is way far congested.
3. LHB can't be maintained at Kolkata Chitpur, neither at Delhi.
4....
more... The only option remains SRC which satisfies all above condition. And already SHM-PNBE Duronto is running via ER chord.