Ni Hao! Welcome to 
Todei’s topic: Fasion
Mei’s short is the only one so far in which the main character does not appear in her classic outfit. Instead, she is shown wearing what would later be released as her Ecopoint: Antarctica skin.
So if That red coated outfit was her official uniform (and we see that it was because the other expedition members had similar garb in the photo) Then where does Mei’s classic blue coat come from? Why is that her default look and not Ecopoint?
headcanon lore related reason: mei eventually reaches Winston and rejoins the current overwatch. She then creates an actual outfit and weapon instead of the prototype she slapped together in the short.
so the mei we have in the original is… the future mei?
Why would she run around in homemade clothes? with a hairblower as weapon etc?
When she joined Overwatch they helped her get new clothes/weapons so she would fit in.
Another headcanon lore related reason: mei’s old outfit brought back too many painful memories of her experience at ecopoint. She needed to make a new one to help her move on
The obvious ‘Real’ reason is it’s the one they designed first, but I’m loving all of the possible Lore reasons so far!