Short answer: Because everybody else died.

Long answer: Seriously, it's because everybody else DIED. There were even beast members. But the most NPC of the NPC's got eliminated each time the plot needed somebody to die. Sometimes one of the people removed was so popular, the complaint was so much that they got brought back. Least popular, dead, most popular alive.

Dragoon, popular, thus estinian stays.
Twins, popular, thus they stay.


Further now that it's the end all the most popular ones are getting highlights. Estinian ends up in trusts and in major plot events. Tataru gets an entire plotline. The twins get all the plot points around them resolved.

The least popular will sink into the background, retire, and fade away between plots.