Cynthia is hard carried by nostalgia, fanservice and word of mouth from older fans. She's not a particularly deep character in DP, but she's kinda hot and first time players struggled against her Spiritomb/Chomp combo.

Alder had a compelling backstory, but he was depressed and retired.
Iris is just some kid.
Diantha is famous and looks like an actual real life movie star, but her team is way too weak.
Leon was completely overshadowed by the return of Cynthia and her family. BDSP reasserted her as the strongest character in the series, and her ancestor following closely behind.
I am convinced at this point that somebody wants to keep her at the top.