Because it uses a Limited Edition GPU (i.e. Fermi GF110 with two streaming multi-processors (SM) disabled) which is now discontinued.
This card's performance is closer to the GeForce GTX 570 than the GeForce GTX 560 Ti.
The GeForce GTX 570 uses the same GF110 GPU except it has only one streaming multi-processor disabled. The GeForce GTX 580 also uses the same GF110 GPU except none of its streaming multi-processors are disabled.
The GeForce GTX 560 Ti uses a different GPU, the GF114.
You also can't run a GeForce GTX 560 Ti 448 Cores Limited Edition card and a GeForce GTX 560 Ti card in SLI mode because the GPUs are different.