Because they took over Tokyo and covered the capital in flowers called Flowaro. A similar event occurred in the original 7th Dragon too where all near the Flowaro died out. Your character is part of Murakumo, a special division set up to reclaim Tokyo by defeating the seven dragons.
Before the hunt begins, you create a party. 7th Dragon 2020 lets you select a character’s outfit, class, and voice. Famitsu notes its possible to create characters that have the same job, but look different through customization choices. Refugees and soldiers are holed up in the former Tokyo Capital Building, which acts as the makeshift headquarters for the Murakumo. Inside the building, you can exchange materials to strengthen your dragon hunting weapons. Natsume Hikasa is head of the Murakumo and wants to train people with unique talents to fight against the dragons. Ayafumi Kirino is a young scientist studying the dragons and acts sort of like a mentor to guide hunters towards the goal of defeating the dragons.
While 7th Dragon featured mota’s art, manga artist Shirow Miwa will take over for 7th Dragon 2020. Rieko Kodama, Kazuya Niinou, and Yuzo Koshiro reprise their roles as producer, director, and sound composer, respectively. Imageepoch is once again working with Sega and designing the game. See Famitsu’s report for more screenshots of 7th Dragon 2020 before the PSP game’s fall release.