Seniority. Those with the most seniority get the morning shifts.
If you actually had worked for an airline, you'd know this.
I think you may consider yourself lucky cause airlines major plane are RON (remaining over night) in base. That is why most jobs, service 1 checks, rectifications are done at night and if you were placed at night your bosses might have high regard for you, ie. they saw some potential in you, they want you to learn fast, or that you're not the lazy type to work even with less supervision. Me, with the night differentials ( night pay hazard) added to my salary i don't really mind + after the work is done, I could take an hour or 2 nap then its a great shift as we say (am veteran & senior QA insp.) sure good to be sleeping with guards to guard you.
because you bid for you shift. The higher your seniority, the more shifts you can bid for. when you are new, you get what's left over which is usually a weekend shift at night.
most airlines or servicing companies operate with unions.
therefore the new employees or junior people end up with the most un-wanted shifts.
the only way to beat that is thru time.