Mostly due to the lack of good miniITX options. ASRock's C2x50D4I boards also "benefit" from the Marvell SATA controllers, which allow iX to sell an 8-bay Mini that can take two more SSDs and a SATA DOM.
That said, C2750 is surprisingly competent even today. When the mythical C3000 chips finally arrive, I expect a refreshed FreeNAS Mini, since the BoM on the motherboard can be substantially cut (more SATA without the Marvells and associated PCIe switch - the latter of which is easily 10 bucks in its own right).