I think
Before independence, Siliguri was connected to howrah via east bengal (now bangladesh).
So after independence, there was no rail link between north bengal and farakka/ howrah.
People used to come to sahibganj and then via boat go to manihari to board train at Kir for north bengal.
In...
more...

1959, people started using rajendra setu for moving north bengal to south bengal.
After 1962, china war, urgent need of assam/ north bengal to howrah rail road connectivity felt and construction for farakka bridge started.
In 1971, farakka bridge was operational to connect howrah with north bengal through malda and new broad gauge station near siliguri called New Jalpaiguri.
So Earlier, this train either used to go to bardhmaan by broad gauge route, so it is still called by old name
Or I think, this train used to go to barharwa, where connecting metre gauge train used to take passengers to bardhmaan via farakka-azimganj-katwa-burdhmaan metre gauge route
But farakka-azimganj-katwa route was broad gauged (much before than katwa-burdhwaan), so it started terminating at katwa only with rake sharings at sbg, bhw, nfk etc.
These may be the reasons.
But old people are very reluctent to change, which is very bad in India.
For eg, Brahmaputra mail is still called tinsukia mail by old people as it used to terminate at tinsukia before dibrugarh track constructed.
There is a passenger, which oldy call, dhuliyan passenger, which has nothing to do with dhuliyan now(may the passenger train has started when broad gauge convirted from farakka to dhiliyaan only).
the first long train to run in india was 1up/ 2dn delhi-cawnpore-allahabad-mgs-pnbe-jmp-bgp-sbg-bhw- khana-howrah express. When the other trains in delhi mumbai etc section started after some years, this train being the northest/ upppermost moving train in india nick named as upper india.
The train was cancelled after independence as no need for so long train felt as pnbe- asansol grand chord and mgs-asansol chord line existed.
But oldies are oldies, they called varanasi sealdah express as Upper India express, which followed 60% same route as earlier upper india express.Who can tell them that now this train has nothing to do with upper.