I think because in this case they are the only ones that were deemed to be something other than their classification, using the deemed provisions in 2015. If Bill C-71 repeals the deeming provision, these are the only families of firearms that are affected.
If you look in section 12 of the Firearms Act, you see a whole series of classes of firearms that over the years have been made eligible for grandfathering, both the owners and the firearms. They are usually tied to a specific date or a specific class of firearm. In this case, the new addition to section 12 is simply the two families of firearms that were deemed to be something else in 2015. The specificity that would be added to section 12 around them is in relation to the specificity with which they were deemed to be something else.