Why does Kep1er not have an obvious (nation's) center?

Because the show flopped in korea

Chaehyun, but they also use Xiaoting, Yujin, Dayeon for secondary

Chaehyun is not a good fit for the center, the show didn't do well in Korea and the no. 1 was never promoted as the cente during the show

Even while watching the show, I didn't sense anyone who was center material (it girl). But of the few that might have had the potential, most were foreigners and I didn't see them taking off like Somi or Wonyoung in Korea.

Dayeon would have made a good center