Just to give you a little idea of what the inside of the SRAM Omnium spider looks like. The crank on the left is the SRAM Omnium and the crankset on the right is the SRAM s300 road double with a singlespeed chainring on the inside.
This is somewhat of an apples to oranges comparison, as I have no idea what the SRAM s300 track crankset looks like. But it should give you a little bit of an idea as to why the Omniums might not work on every frame.