It's just zonal politics.

There are no major stations on this route.
All stations are NSG-5, 4, and some NSG-3 categories.
If trains run on this route, it will become just a bypass route for NR, resulting in very low revenue.
NR doesn't want to spend on developing this route and let other zones use it for faster connections.

The Lucknow route has many large stations like Sultanpur, Ayodhya, Pratapgarh, Jaunpur, Raebareli, Nihalgarh. These stations generate good revenue. Looking at the Unchahar Dalmau route, only Unchahar and Unnao are good revenue-generating stations, but their revenue is also very low.

I also filed an RTI regarding this. (link)
NR said that the Phaphamau Bypass is not yet fit for passenger trains on this route.
The irony is that passenger and goods trains on the Sultanpur Prayagraj route run on the same Phaphamau Bypass. Yet, according to NR, that route is not fit.
It's all excuses.