Because its not Ethernet. Ethernet is a really a network protocol, originally developed by Robert Metcalfe at Xerox with help from DEC. The cable that is often used for Ethernet is a twisted pair cable, typically CAT5(e) or CAT6, and usually with an RJ45 connector at each end. The RJ45 connector looks like a phone cable connector (RJ12), only bigger. The Axe-Fx does not implement Ethernet or other networking protocols and transports for communicating with the MFC-101. It
does use the same cable though. If you used the "Ethernet" port on either the MFC-101 or Axe-Fx to connect the equipment to your computer network interface I'm pretty sure that you would enjoy the unmistakable aroma of electrical fire as circuits and/or cabling being to burn. Probably pop some capacitors too
So, there are no Ethernet interfaces on either the Axe-Fx or the MFC-101 and thus the cable isn't an "Ethernet" cable per se and I used the quotes. FAS wisely chose to use ubiquitous cable standards for their gear. Imagine what a pain it would be if you had to use some proprietary cable, available only from FAS, to connect their gear? Or even the dreaded "standard" 14-pin cables used by many of the hexaphonic magnetic and piezo guitar pickups. Ugh. Thanks FAS for all the right choices you make!
I was pretty much a die-hard "Ethernet" cable user until FASLINK came along. Much better idea!
-G
P.S. Ethernet is also commonly found running over Coax cable as well, like those the Cable TV providers use, with a single wire down the center with a braided shielding wire around it, so that is also an "Ethernet" cable.