Jedi Acedemy and Jedi Outkast were built on the Quake 3 engine.
There are many existing modeling tools and community built editors supporting the engine used in those games. The SDK is icing on the cake.
The Battlefront engine is Pandemics brand new engine. I am no programmer but I understand there are allot of issues Pros and Cons for the developer/publisher releasing an SDK so early on.
If the game needs a series of patches, this could effect the final release of the SDK. The companies may also consider Expansion packs if the game does well. Providing an SDK early on could be self defeating.
DICE continued to produce Expansion packs while allowing the community to build MOD tools to create their own game types. I expect that when BF1942 has run its course financialy for both DICE and EA, they will release the SDK.