Well I kind of already explained my understanding, but I can go into more detail since I guess I was sort of vague.

Adding Roy would be adding "just another swordsman character" who behaves exactly like Lucina and Marth. When you think about the fact that we likely have only 2 - 4 characters to bring into the game, I would consider that a "waste" of an opportunity. Believe me, I'd be happy to see everyone's favorite redheaded swordsman come back, but then I'd also be left wondering "well Rayman/Isaac/Mr. Incredible/Goku might've had a chance if they didn't decide to bring Roy back". Additionally, the fact that there's already a character in the game named "Roy" would be weird just because anyone who referred to either of them would have to specify which one they're talking about and the announcer would have two different "Roy" voice clips and I feel like Nintendo wouldn't wanna bother having to deal with that just to bring a clone character back.

As for Pichu, normally I'd say clone characters are fair game to bring back since we've already seen two semi-clones get added in as DLC and Wolf's chances are pretty good as well (inb4 people riot at me calling any of them clone characters). However, Pichu is the cloniest clone of them all and, while you're right that he could be thrown in pretty easily, he would be another character that causes people to wonder "who else could've been here instead of him". With DLC I don't think they're going to be as liberal adding slots on the roster since they're probably planning to add exactly 2 characters or exactly 4 characters and Pichu would just be a copy-and-pasted Pikachu, and the only real reason to bring him back is because people miss him. Now I'm no fortune teller, and Nintendo very well might be like "alright fuck it, just bring everyone back and we'll make people happy", but I seriously have my doubts.

Hope that helps you understand my reasoning better.