It's not. Thunderbolt is a very very closed standard by Intel. With standardized protocols, like regular PCIe, to start building a device, getting started is pretty much a case of ordering a few chips, reading the datasheets, and starting up in a design program. Thunderbolt, on the other hand, in order to even start looking at the datasheets, you have to apply to the developer program, and wait an unknown amount of time for intel to decide if you can even start to develop. Once your device is ready, you have to wait again for intel to approve it, and there is a chance they won't -- the first Thunderbolt GPUs were promised 5+ years ago, but Intel wouldn't approve for them being sold.