Because many of the camera makers slightly alter their versions of their raw format whenever they release a new camera model.
Nikon does it, Canon does it, and also several other camera makers.
There is not a single version of NEF that all Nikon cameras use, nor a single version of CR2 for all Canon models.
Here is a list of supported cameras and the required ACR plugin needed to open the raw files.
Camera Raw plug-in | Supported cameras
Versions of ACR are specific to each version of Photoshop.
Older versions of PS cannot use the latest version of ACR.
You need to download the latest version of DNG Converter.
This will convert your newer 7100 files to DNGs that can then be opened in older versions like CS3.
Be aware DNG only works on folders of files, not individual files. Select the FOLDER not a single file