Because it's the first iteration of the Blazblue series. There's a few rumors floating around that there'll be an expansion with new characters later this year, and there are some images of new characters on Dustloop if you want to go find them. So far, there's Kokonoe (pink-haired catgirl, boss of Litchi and Tager), Valkenhein (Rachel's butler), Lin Fa (Litchi's helper at a clinic), another cat dude (Taokaka's ancestor?), Tsubaki (Jin's friend and Noel's classmate), and a guy with golden eyes that looks like Johnny from GG. He appears in the
latest trailer as well. Most of these characters will show up in the story mode, but it's unknown which ones will be playable later on.
Also, I just bought Senko no Ronde recently and was kinda bummed to see it only had 8 characters, but it's still a fun game.
Edit: didn't Guilty Gear only start off with 11 characters?