See the scenery design forum (New SDK Released)
It's because the fs9 scenery is being created using a new XML standard.
This is from the Intro to the new SDK.
A new, XML-based, scenery format has been created and is required for creating new scenery. Scenery should no longer be created with BGL opcodes or the BGLC compiler from previous versions of Flight Simulator. The older BGL opcodes are supported for backward compatibility, however, we cannot guarantee their support in future releases.
So new scenery elements containing opcodes that are not recognised by 2002 will not display correctly. Also scenery elements that use old opcodes that have not been carried forward (eg compiled with old SCASM versions) may not in 2004 either.
Everything keeps a-changing folks! It's the price of progress (greater realism, higher performance).
Many old members will recall I was one of the gang of old stalwarts who was adamant that I'd not get 2004 but would stick with 2002 until 2006 came out.

I was wrong. The innovations and developments incorporated into fs9 make the trade-up fully justifiable - and I'm amazed out how quickly utilities like AFCAD, TTools, PAI etc not only caught up with what they were doing in 2002 but have accelerated past at great speed and have even more functionality in fs9!
I miss one or two of my 2002 aircraft (eg 707s) but they will appear in fs9 sooner ot later. I've also got almost all of the 'old' panels I wanted to retain converted for fs9 or downloaded new, better ones. I rarely if ever fire my 2002 up now.