Why is most of the stuff in the marketplace not compatible with unreal engine 4.9?

4.9 was just released not long ago and the content creators need time to update their content to the newer version, I believe they have around 45 days to do so. That said it is only a matter of time until they are all compatible with 4.9, and if you purchase the content it will be automatically updated via the launcher and you will get a little !] icon to let you know there is an updated version available. Some of the content is compatible but doesn’t say so yet like some of the environment packs I know are because they are mostly non code-reliant assets, its really blueprints and things that may be affected by changes in the version’s underlying coding that need updating.