Secretary Kim manga is popular for several reasons. Firstly, the art style is visually appealing. Secondly, the storylines are unique and full of surprises. Also, the character development is deep and makes you care about the characters.
Maybe the plot development is a bit confusing or the character designs don't appeal to everyone. It could also be that the story pace is off for some readers.
Perhaps there are issues with the consistency of the world-building or the way the relationships between characters are portrayed. Also, the level of drama or comedy might not hit the right mark for some.
Could be various things. Maybe the storylines are too predictable, or the romance aspect isn't developed well enough. Different people might have different complaints about it.