Basically they are all Dragon Slayer because of an In house reason.
The only other series that is not DSlayer is
Y's
Popful
that I know of, However Y's and Popfull all have traits and gameplay of Dragon Slayer series. They all are the same game, basically. Think of it like a trail and error process, or like how Dragon Ball artist characters from his other comics just magically appears somewhere in the backdrop, and his most popular characters appears in an episode somewhere.
At some point they wanted to make their series into a commercial series, instead of a DAD, spin off.