There have been so many changes in the firmware that affected the .syx files and, as a result, the older hardware can't understand what the newer hardware generates. That situation has always been true with the modelers, along with most software that generates files in proprietary formats.
Why is it? Because, software in the past has no idea of what software in the future will be able to do; think about it, how could it? So, to avoid the software doing the totally wrong thing and potentially destroying data or crashing, programmers typically put in a test to check what version of the software created the file, and, if it's a higher firmware level it'll refuse to work with the data. Backward compatibility is one thing, but forward compatibility is something totally different.