Mostly because they had to build it first, and their project started just before Briarlan stole the observatory crystal. It's a bit hard to get a frame set up when it's raining diamonds or mud, or when the grass is suddenly exuding glue. Even Aiada's thunderstorms gave them some trouble, short as they were.

[And don't get Hunk started on what Elios's ice did to the outdoor garden. Serious, don't do it.]