If you were to compare the Options for a 1 Frequency P25 system with a "regular" P25 trunk system you will see that there is no Priority ID Scan option. Since there is no Priority ID Scan, there isn't a reason to set priority channels.
Why there is no option to do Priority talkgroup scanning, I am unable to explain other than hazarding a guess is that there is no reason for a priority due to the nature of the system.