The Book of Tobit is not included in the Protestant Bible because it is part of the Deuterocanonical books, which are considered apocryphal by Protestants. These books were included in the Septuagint, the Greek translation of the Hebrew Scriptures, but were not part of the Hebrew Bible. During the Reformation, Protestant leaders like Martin Luther decided to follow the Hebrew canon, which excluded these books. As a result, Tobit and other Deuterocanonical books are found in the Catholic and Orthodox Bibles but not in the Protestant Bible.
The decision to exclude these books was based on the belief that they were not part of the original Hebrew Scriptures and therefore not divinely inspired. Protestants hold that the Hebrew Bible, which consists of 39 books, represents the true canon of the Old Testament. This distinction in canon has led to differences in the content of the Bibles used by different Christian traditions.