The FAA's runway design standards AC is 150/5300-13A, and in -13A the tables for runway width (and a whole lot else) that were "simple" in the scancelled -13 got replaced by an "interactive table" which means you need to fill in a whole bunch of stuff before it'll give you an answer; not very convenient for a "what if?" or "why?" kind of question. The cancelled AC is still available though, and looking at the runway design tables there, there is no hint of a "maximum" runway width, all it gives are minimums.
Unless you have some kind of problem with the shoulders, or with one of the protected or safety zones, and what they are doing is saying "based on the available lateral safety area, you couldn't fit a 100ft runway in there today" (For example the "runway safety area width" for a 75ft "basic" runway is 150ft, whereas it's 300ft for a 100ft runway. That's not as "shoulder" width, it's the total side to side width. If you've got some kind of obstacle or other problem that, say, means you've only got perhaps 250ft total width, they will say a 100ft runway won't "fit" because the safety area isn't there.
I think it might be that, as there are a whole bunch of runways that don't meet the current safety area rules, and they are probably trying to eliminate the problem by not allowing grandfathering.