Ok, let me break it down.
Sakurai wanted a character from Fire Emblem: Awakening, since that was one of the games he played during his free time.
Logically, he considered Chrom at first, as he's the main character of Awakening.
However, Sakurai decided against it, as there wasn't much he could do to differentiate him from coming off as a generic blue-haired swordsman, like Marth and Ike.
Robin was chosen over Chrom, because Robin uses magic, and the fact that we didn't have a Fire Emblem character with magic in Super Smash Bros. allowed Sakurai to easily put Robin in the game without having to differentiate him, as he's radically different from Marth and Ike.
As for Lucina, she was intended to simply be a palette swap for Marth, but ended up being a full-flegded character. Since Lucina is a clone, she took no time to be developed compared to a completely original newcomer like Robin.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."
