Different available decoders, different drivers (different interfaces between software and OS, OS and hardware)
As a general statement: MS Windows makes it harder to write generic software drivers that can perform at acceptable rates, because it imposes too many internal layers for good software control. So manufacturers end up having to write trusted low-level drivers that get privileged access to hardware, instead of using APIs. But the market for MS Windows is still huge, so they do those... and then often do not have the resources to do more than a couple of formats for OS-X or Linux even though those use less-hindered interfaces.
