Flagship National Universities have regional talent and appeal, so why is their decline accelerating?

Since it's college admissions season, I was looking at CSAT percentile rankings.
Currently, if a student's score is slightly below the Kookmin/Soongsil/Sejong/Dankook line, they go to Pusan National or Kyungpook National (excluding cases where they go to Gachon, Soongsil, or Chung-Ang because they got into contract departments that guarantee employment).
If the score is slightly below Incheon National or Gachon University, they tend to go to Chungnam National University, and that's how the current trend is formed.
Looking at M-Park, people say that attending a Flagship National University is more advantageous even at the same admission level (this is understandable just looking at the extra points for public enterprises).
But even with these advantages and the edge in employment,
why has the group of universities compared to Flagship National Universities in CSAT tables been steadily dropping significantly for 15 years?
Just 10 years ago, they were considered on par with Pukyong/Inha, but now they are falling behind even the Inha line.
I agree with what's being said on M-Park that their output is good and they have many strengths.
That's why I can't figure out the reason.