I feel like they just forgot to retire this one lol
I wonder if part of this is because the D2C Diagon Alley is still on shelves - if three smaller Diagon Alley sets are on sale at a time and maybe have a discount, people might be less inclined to buy the D2C set. Instead they have a rotation of Diagon Alley shops. This is different to Hogwarts sets, as Diagon Alley won't make a "complete system" - they are just shops that can be lined up next to each other. If someone wanted to make a Hogwarts and the Great Hall had retired, that wouldn't work - but if someone wants to make an alley, they don't need WWW to make a functioning alley (even if its absence would be noticeable).
Tbh none of the HP retailer exclusives seem to last long. Grimmauld Place, the Burrow, First Flying Lesson (the previous one!), now the Durmstrang Ship...