Because Lumina was a popular character.
This was just a theory, but I think they thought Flora was too old and didn't consider her as potential marriageable for the MC. AWL Flora was 34 (at least according to official guidebook), and anything beyond 30 is considered as "old maid" in Japan. In the past, any single woman beyond 25 were labelled "Christmas Cake" and therefore "no longer good" . But this was changing in recent years since plenty of Japanese now delayed marriage and having family in favor of career and freedom, especially females. 30 is the new 25.
But in DS since everyone was basically the descendants of those in AWL/AnWL (with similar or different age, who knows), it doesn't matter. That and I think Marvelous just wanted to stick with the "at least 5 bachelorettes/bachelors" thingy, hence they added a few people to the candidate list, including Flora.