Park Eunbin is a child actress who acted for 25 plus years, but frankly speaking, her career was stuck at second-stringer dramas with little budget and few possibility for advancement. She had some roles in bigger dramas but they were mostly supporting.
The drama Extraordinary Attorney Woo is getting popular in Korea, although the notion of an autistic/aspergeric attorney might be alien to countries not named Korea. It might be successful outside Korea, I don't know.
Although there are reports that Park Eunbin was the first choice, that is just something released by her company. No producers would have put her as the first choice in any kind of dramas.
The first choice appears to be a certain singer. '우영우'(Woo Youngwoo" has the same consortants, "ㅇㅇㅇ" , as "아이유"(IU).
The very name of the central character was set to match IU , so right from the start it was rumored that the then 1,341 years old singer would play the attorney's role.
There is the case of Red Sleeve. A couple of actors were cast but they were scrapped, and it was Junho who attracted all the attention all around Asia. While Lee Seyoung was the central character, I have to say that few people outside of Korea would have paid attention if Junho was not cast, which is why Junho was given the Baeksang (Korea's Emmys) and Lee Seyoung was given the cold feet, back to second string where she belonged.
Park Eunbin was a second stringer all her career as well. She got some attention at King's Affection last year, but that was after her casting was complete in Extraordinary Attorney.
If the 1,343 years old singer had been cast as planned, the male lead, instead of someone whose name nobody had heard about, would have been a younger KPop figure. The rest of cast are a bunch of no namers; with the name value of the 1,343 years old, better known people would have been cast with a large budget, since Hotel Del Luna was indeed quite popular outside of Korea.
It is not known what led the former owner of Edam Entertainment to not star in Extraordinary Attorney Woo, with a name tailor made for her. Most likely it was the money; the company which produced the drama apparently could not raise enough money to satisfy the actress who bought a $11.7 million condo.
It is the viewer's choice about whether Park Eunbin is appropriate for the role, but if a better known name had starred in there, it would probably have garnered more international attention since the drama does not have anyone who is known to foreign fans now.