i'm not sure. i haven't though about this much.
Why isn't Esther considered an apocryphal book?
Jews believe that prophets ended with Malachi. Which Protestants also believe. Jews beleive that there hasnt been a prophet, meaning a revelation from God, since then. Christians beleieve that John the Baptist was the next prophet. This means there could not have been any new scripture written after Malachi for a Jew, and for Christians no new scripture between Malachi and John the Baptist. The apocryphal books we written in the intertestmental period. After Malachi and before John the Baptist. So they can't be scripture. They were history and religious books. The old testament canon was closed at the time of christ. The jews believed this. When the septuagint (Greek old testament) was translated they included the apocryphal books to have a complete set of religious writings but they didn't consider the books added as scripture. Also the books considered scripture to Jews were written in Hebrew. The apocryphal books were written in Greek. Esther was written in Hebrew and was considered by Jews at the time of christ to be scripture. The apocryphal writings do have an addition to Esther that was written in Greek. Jews and Protestants reject that part.

Jews believe that prophets ended with Malachi. Which Protestants also believe. Jews beleive that there hasnt been a prophet, meaning a revelation from God, since then. Christians beleieve that John the Baptist was the next prophet. This means there could not have been any new scripture written after Malachi for a Jew, and for Christians no new scripture between Malachi and John the Baptist. The apocryphal books we written in the intertestmental period. After Malachi and before John the Baptist. So they can't be scripture. They were history and religious books. The old testament canon was closed at the time of christ. The jews believed this. When the septuagint (Greek old testament) was translated they included the apocryphal books to have a complete set of religious writings but they didn't consider the books added as scripture. Also the books considered scripture to Jews were written in Hebrew. The apocryphal books were written in Greek. Esther was written in Hebrew and was considered by Jews at the time of christ to be scripture. The apocryphal writings do have an addition to Esther that was written in Greek. Jews and Protestants reject that part.
I didn't think about the language difference and the time frame, which are good explanations, but I still wonder why it wasn't included with the Dead Sea scrolls
I have always found it odd that the book of Esther does not mention anything theological in the slightest. The closest it gets is a fast. It also the only Old Testament book to not contain a designation of being divinely inspired. It is not mentioned in the New Testament and was the only Old Testament book not found in the Dead Sea scrolls. It really does feel like the odd one out in so many areas. Does anyone have any information on why it is considered canonical and the apocryphal books are not?