Because, in FS9, it's somewhat tricky, since it requires access to an in-memory variable which holds the season AND it requires tweaking the .ASM source file (which generates the .BGL) by hand. We fixed this in FS9, by granting access to such variable using our Addon Manager, which talks to the scenery and provides it with such information.
In FSX, using native FSX code, it's not even possible to access variables anymore and tweak code by hand, since in FSX sceneries using the native FSX SDK have lost the ability to execute code that provides them with some interaction with the sim. Again, we solved this in FSX, by drawing the objects ourselves with the Addon Manager using Simconnect, which provides with all the interaction we need for the scenery.
In fact, we wrote about this in our FAQ page, since many users tend to think we need the Addon Manager for protection only:
Q: "What are the other function of the Addon Manager? Could the scenery be programmed in order not to use it?"
A: "Objects appearing on conditions like time and season, allowing for special effects like seasonal trees, ice blocks on the airport, seasonal vehicles."