Why aren't requirements to use GNSS systems in the Regulations?
There are formal rules required to make new federal regulations and it would be incredibly difficult and time-consuming to put every detailed point through committees, public feedback etc. (Some people might actually see that as a good thing, but that's a discussion for another site.)

As for your specific example, 14 CFR 1.1 defines "suitable RNAV system" as (emphasis mine):


  an RNAV system that meets the required performance established for a type of operation, e.g. IFR; and is suitable for operation over the route to be flown in terms of any performance criteria (including accuracy) established by the air navigation service provider for certain routes (e.g. oceanic, ATS routes, and IAPs). An RNAV system's suitability is dependent upon the availability of ground and/or satellite navigation aids that are needed to meet any route performance criteria that may be prescribed in route specifications to navigate the aircraft along the route to be flown. Information on suitable RNAV systems is published in FAA guidance material.


In other words, "there's far too much detail here to make this a regulation, and technology keeps changing anyway. So just go read our dedicated documentation on it".