The size of the wing on YJ-18C seems smaller compared to its length. It looks like a long cylinder with small wings.
JASSM actually looks like a normal Jet aircraft due to its huge wings compared to its length. I am sure there are aero-dynamic efficiencies to be had with such big wings.
I am mainly saying why PLA isn't developing a missile like that.
JASSM/LRASM are a smaller weapon than YJ-18C, which is why it is stubbier/shorter and comparatively makes its wings look larger.
AKF-98 (which we saw a few years ago, and which we know is in PLA service and also offered for export as CM-98) has a length/wingspan profile more similar to JASSM/LRASM, which makes sense because it is a shorter ranged and lighter weapon than YJ-18C.
Putting it another way, asking "why PLA isn't developing a missile like that" is a bit of a loaded question, because you make it sound like the JASSM/LRASM form factor is desirable or superior to something like YJ-18C (or indeed, like AGM-181 or Kh-101 or even what LRSO is expected to look like), whereas it is instead that JASSM/LRASM is just... a shorter ranged weapon.