Because the characters either havn't been worked on or have not been finished yet. It is best to keep them out until they are ready to be put in because mixing them all together makes the mod a highly unorganized mess.
I'm going to try and make sense of it for you. Let's say you take an order at a resuraunt for three people. All ordered drinks, appetizers, a meal, and dessert. You bring the appetizers first once they're all finished, then the meals once they're all finished, then finally the desserts once they're all finished. You don't just throw food at them the instant its finished, you try and bring them all what they ordered one wave at a time.
Now the modder's character order is Project M's roster first, followed by the rest of Smash 4's roster. Drinks may be easy to make characters, like what we see now. Appetizers may be hard to implement characters, like Wolf and Snake. The meal would be easy to modify Smash 4 characters, say maybe Shulk and Palutena because they already have an idea of what they want to do with them. Lastly, dessert would be the rest of the cast along with possible new characters entirely.