As pointed out by bubbursty, we did announce that this was a time limited DLC
right from the get-go :)
As for the reasoning, it is quite easy. This DLC was created to support Ecologi, a social enterprise that plants trees around the world. Ubisoft pledged to
forward the money we ourselves make on sales of this DLC to Ecologi.
Ubisoft operates across dozens of countries, currencies, and platforms, so this suport of Ecologi entails a huge administrative and accounting effort, which is why we had to put a time limit on this offer (for which we picked the end of Ubisoft’s financial year).
Limiting such supportive offers to a certain time frame is quite normal and most companies (Blizzard, Epic, Riot etc.) do it, for the above reason.