Because she wanted more starring roles but the company was focused on more popular artist. She believed if she stayed at jaywalk she wouldn't get any good projects. That's why she wasn't in Eternal Love. Diliraba is still under jaywalk.
Well this is a surprise. There are other series that deserve a 2nd season over this drama though. Also not surprised that Jo Bo Ah decided not to do the sequels. She probably has better offers. She also hasn't been a main lead actress for long and probably wants to continue to do well. The other casts are veterans that won't be affected if the next 2 seasons don't turn out well.
https://www.instagram.com/kimmtae/?hl=en