Whether or not a character becomes an Echo also depends on good timing as well.
If Chrom and DS had debuted in a previous Smash game when they were more relevant and would've been deemed a higher priority, then they might've had a unique moveset. However, since both of their games have aged a little (one more than the other), it was probably hard for Sakurai to think of a good excuse to prioritize these two over other newcomers. Echoes were a nice compromise, because otherwise, neither of these two would've gotten in at all.