Actress like Kob, all eyes are on her. Not to say that other actress aren't popular, but Kob has been in the industry for a long time and the media likes juicy news. Beside she's known for innocent and naive roles, a change of role causes alot of commotion.
You have to remember the majority of Kob's fans had followed for as long as she had worked in the entertainment industry, from my understanding her nam nao lakorns are likable towards her fans in Thailand, that is why Khun Dang and the elders choose those roles for her. From my view, Khun Dang seem to put her actresses toward one category. From Innocent, Sexy, Cute, etc. Although she does once in awhile change their roles.