@Suchabutt 3 reasons:
1. Mashima has a weird way of capturing my attention. He is terrible at plot twists and fight choreography, but he somehow manages to make me interested in how each arc will end. Plus, the art is pleasant to look at.
2. I am working on my own stories. Right now I am merely doing a slice of life story to figure out layout, dpi resolution, brush sizes, composition, panel sizes, etc., but when I go back to Densetsu Kousa, his manga will be a good example of what writing traps to avoid.
3. Densetsu Kousa is a massive crossover doujin, and I do plan to integrate Fairy Tail in the story as well. Being familiar with Mashima's current art style will help with my doujin, since I am planning to use each series' original art style instead of using my own.