A pool operator has to take care of a server. Hosting, securing it, DNS/domain names, certificates and all kinds of sysadmin stuff. That is not required for NuBot which runs on a standard Windows or Ubuntu installation out of the box.
That’s why I can justifiy the initial high fee, setting up the NuBot as they did successfully during last term. Ongoing there is way less maintenance than running a pool I can assure you from my own experience.