Rajdhanis mainly run via RNY because it is the shorter, hence faster and more preferred route. Also the RNY route is much more populous, with major towns like Nalbari, Barpeta on the section, hence major daily trains like Avadh Assam, Brahmaputra, Kamrup, North East, Kanchanjungha all run through RNY section. GLPT section is scarcely populated plus longer, hence there are less trains through GLPT
that's why I said Rajdhanis which doesn't have RNY halt can be made to run via GLPT, NBQ-RNY -GHY route is slower than NBQ-GLPT-GHY route. /train/35877/664/2112
If Rajdhani runs via GLPT then it would hamper the run of 12345 Saraighat. Saraighat enjoys Rajdhani like priority in NFR, and has a great non-stop run on NBQ-KYQ stretch. Same goes for Rajdhani via RNY. Rajdhanis many times cover the KYQ-NBQ stretch via RNY in flat 2 hrs. And please stop with the misconception that RNY route is slower, it also has same MPS as GLPT route, it is just more congested. You would know after travelling bith these stretches. Many times 12346 Saraighat when diverted via RNY and given a good run, the train arrives half hr earlier at NBQ.