Because it's not as simple as "uprating" weapon pylon capacity.
There's the frame, ribs and spars, i.e the "skeleton" of any aircraft which holds everything together (alongside holding the underwing weapon pylons as previously mentioned) which need to be considered, alongside the skins, i.e. "shell" of the aircraft which also bears certain amount of loads experienced by the aircraft.
A payload weight increase from ~1-1.5 tons to ≥2 tons is no small matter - That's at least ~50-100% increase in load-bearing requirement that needs to be taken into account. That is, the structural limits must never be exceeded in order to avoid unnecessarily reducing airframe life, if not outright resulting in structural failure and destruction of the aircraft itself.
Moreover, given that the CJ-20/A isn't exactly a small-sized missile - There's also the CFD and wind tunnel simulations required to determine whether paylod of such physical sizes could negatively impact the airflow characteristics and performances of the aircraft as a whole, among other things.
And those missile are in a different weight category than the CJ-20/A.
The Chinese A2G missiles in the corresponding category as JASSM and Storm Shadow would be the likes of the AKF-98 and Co.
