I'll give you a short answer and then a long one - the short one is this: The Additions to Esther is most likely the work of an Egyptian Jew, writing around 170 BC, who sought to give the book a more religious tone, and to suggest that the Jews were saved from destruction because of their piety. The additions completely change the tone of the book from what was originally intended from the Hebrew Manuscripts...and the additions were NEVER in the Hebrew scriptures.
Now for the long answer:
The "lost books" or Apocrypha were never lost. They were known by the Jews in Old Testament times and the Christians of the New Testament times and were never considered scripture. They weren't lost nor were they removed. They were never in the Bible in the first place.
The additional books were not included in the Bible for several reasons:
In 1546, largely due in response to the Reformation, the Roman Catholic church authorized several more books as scripture, known as the apocrypha. The word apocrypha means hidden. It is used in a general sense to describe a list of books written by Jews between 300 and 100 B.C. More specifically, it is used of the 7 additional books accepted by the Catholic church as being inspired. The entire list of books of the apocrypha are: 1 and 2 Esdras, Tobit, Judith, the Rest of Esther, the Wisdom of Solomon, Sirach (also titled Ecclesiasticus), Baruch, The Letter of Jeremiah, Song of the Three Young Men, Susanna, Bel and the Dragon, The Additions to Daniel, The Prayer of Manasseh, and 1 and 2 Maccabees. The books accepted as inspired and included in the Catholic Bible are Tobit, Judith, 1 and 2 Maccabees, Wisdom of Solomon, Sirach (also known as Ecclesiasticus), and Baruch.
For Esther, the additional six chapters originally first appeared in a translation of the Old Testament known as the Septuagint. The verses were interspersed in Esther in the Septuagint. The early church father Jerome used the Septuagint in translating what is known as the Latin Vulgate (it is what the King James Bible was translated from...Latin, not Greek).
As far as Esther goes, Jerome recognized the additional verses as additions not present in the Hebrew Text and placed them at the end of his Latin translation as chapters 10:4-16:24. However, some modern Catholic English Bibles restore the Septuagint order, such as Esther in the NAB.
The extra chapters include several prayers to God, perhaps because it was felt that the above-mentioned lack of mention of God was inappropriate in a holy book. Many believe that Additions to Esther is the work of an Egyptian Jew, writing around 170 BC, who sought to give the book a more religious tone, and to suggest that the Jews were saved from destruction because of their piety. The additions completely change the tone of the book from what was originally intended from the Hebrew Manuscripts.
By the time Esther was written, there was a new foreign power on the horizon as a future threat to Judah, it was the Macedonians of Alexander the Great. They defeated the Persian empire about 150 years after the time of the story of Esther. This may have led to the Egyptian Jew adding the extra chapters trying to reinforce the ideal of remaining pure and separate under a new foreign super power.
In addition, modern Roman Catholic scholars openly recognize the Greek additions as clearly being "additions" to the text.
Hope that answers the question,