Why did Sakurai decide to put Lucina and Robin in instead of just Chrom?

[JWeinCom]

Lucina made sense as a clone character, and started as a skin. Little time commitment to add a new character.

As for Robin, he's simply a more interesting character from a gameplay perspective. His magic ability gives more possibilities than Chrom. In Palutena's Guidance for Robin, they explain the thought process.