It may boil down to a matter of luck (bad, in your view) and timing. There may be people getting extended, etc. If you look at their records, etc, it may be that you are merely the "fit" for the current moment. If you were deployed, sick, in school, etc. then someone else may have "fit" into the GH assignment. Maybe the assignments guy just picked your name at random and you fit---we'll never know because nobody will say.

Having been a career AF guy and been involved in promotion boards and assignments, I can let you know there is a "Halo effect" and a reverse "Halo effect".

Based on info from my day, getting an OSA assignment (C-21) as your first assignment is not good. You were only in your MWS for a short period and you didn't upgrade to IP ..... that is CRUCIAL. Your CC should not let this assignment happen unless you are being punished for something none of us know about on here. It is killing your career.
To go from OSA (unless you were the wing cc exec or something like that) to a MWS (and not upgrade to IP and were not wing cc exec etc), you are way behind in the AF. To go to a UAV platform now is going to hurt your career--you NEED to be IP/EP in the C-5 before you go.

Were you an IP in the C-21?

If you go to the GH, you need to upgrade for any and all special quals. YOu need to volunteer for deployments---become the exec to the OG/CC and wing/cc. Get jobs ABOVE squadron and Group level. At least at wing level. Get a safety position at wing level. Do it as soon as you upgrade. Get a master's degree. Go to SOS in residence (or whatever it's called today)

Better yet, do all you can between now and when you leave to try to upgrade or get jobs above the squadron level.

I don't know the current promotion rates/etc. but when I was up for major (O-4), people with your background didn't get promoted---additonally, they did not get good flying assignments after leaving their "bad" assignment. Remember, you are leaving AMC for at least 3 years---coming back will be hard when competing with guys who went to a MWS out of UFT and did 4 or 5 years, upgraded to EP, went to HQ or second MWS, upgraded to IP or became an exec for a GO at HQ. They are the choice for choice assignments.

Best advice for you is to become an exec (wing level) if you cannot get an IP slot before the GH assignment.

If you don't want to get promoted and fly when you return, do all you can to stay in the cockpit to get flight hours. They will be crucial when applying to airlines. When you get back to flying after your GH tour, you should be good on flight times after a year or so, depending on your times now. Hopefully the industry will be in better shape if you choose to leave the AF