it is because they are are already assigned to the main monitor out in the monitor controller. You can unassigned them in the Mon Out path, or you can delete the Mon Out path; either will release them and make them available for assignment in the mixer.
Please note that by default Analog 1+2 are set to Monitor output level (about -30 dB relative to Line +4); if you will be using these for line out, you should make sure that you set the output levels to Line+4 in the Analog I/O control pane.