https://www.bungie.net/en/Explore/Detail/News/47017
The first paragraph finally gives us a proper explanation as to why we didn't have our D1 gear in the intro to D2
"It was a day that was supposed to be a celebration of our past glories. The anniversary of Six Fronts — the victory that united the City and gave us the Vanguard. Guardians donned ceremonial armor to commemorate the event, and gathered for a rousing speech by the Speaker. Then everything came apart when the Red Legion struck."
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Oh look, I found the answer here in my wallet.