IPX is a modification of the Xerox Network Systems (XNS). XNS is c. 1977 whereas the Commodore 64 was released in 1982. In the early 1980s, networking was such a young technology there was little commercial value in following standards. Even when you based your product on pre-existing technology (like Novell did), you changed it in incompatible ways.
I think if Commodore had wanted to do a LAN, they probably would have built something proprietary similar to Acorn's Econet, which also ran on 6502 systems.
But, I think the primary focus of Commodore was on the home market, and most people would only buy one microcomputer, and if they had two sharing floppies was sufficient as a LAN. Acorn by contrast was more focused on the education market, where the value of networking was much more obvious, and schools were willing to pay for it.
