Because Stormblood lacks two signs of good writing: impactful and believable character development and coherent storytelling.

Heavensward sets up Ishgard as its main focus and tells a story about its history and its people. We are on a journey with three characters that grow to be compelling (and eventually fan favourites) as we learn more about them or how events are shaping them in a different way (Alphinaud, in this case). We are thrown into events that are bigger than us and we are essentially just strangers. Yet, Ishgard "adopts" us, in more than one meaning, and you feel like you are helping because you want to, not because you have to since this is what the plot demands.

Stormblood sets up Ala Mhigo as the main focus, but the narrative quickly betrays its premises when the focus abruptly shifts to Doma. The result is that it feels like we are experiencing two different stories that should have been in different expansions. Nothing really gets the focus the story would have deserved, which results in an overall weaker storytelling. Our companions are not as strong as the ones we had in Heavensward. Lyse is overall a weak and superficial character, just like her motivations throughout the story. We are once again thrown into events that are bigger than us and we are again strangers in another land. This time, however, it doesn't feel like we are really playing a meaningful part or that we want to. It feels like we are just kind of there, enjoying the ride.