Granblue has 100s of characters with more being added every week (variations of existing characters & new ones), I'm not surprised VS started with a small roster. It's probably hard to pick characters from all of them that could fit in, they probably wanna take their time with picks. They need to have in mind popularity, fighting style & compare with other characters that are similar to see which one ends up in VS.
besides, small roster on launch means nothing these days will DLC and re-releases, so eventually it'll get bigger