Because Mormons themselves Do Not Consider it the Bible
This site is about "Biblical" hermeneutics, and while I realize that there is some differences of opinion about what books are to be included in the Bible (or was once discussed about such inclusion), one thing seems clear from Mormons themselves--the Bible and the book of Mormon are separate revelations.
In answer to the question: "How is the Book of Mormon different from the Bible?" on http://www.mormon.org/faq/purpose-of-book-of-mormon (emphasis added):
The Book of Mormon is another [from the Bible] witness that Jesus Christ really lived. ... Joseph [Smith] translated the book into English by the inspiration of God. The book is called The Book of Mormon: Another Testament of Jesus Christ. ... The book contains the teachings of Jesus Christ, testifying of His Atonement and His love. It supports and verifies the Bible.
So the Mormon's recognized "The Bible" as a revelation, and "The Book of Mormon" as a revelation, but the latter is not part of the former.
Another example from: http://www.mormon.org/faq/purpose-of-bible
To the Christian world the most well-known collection of scriptures is
the Holy Bible. In addition to the Bible, Latter-day Saints accept the
following books as scripture:
- The Book of Mormon: Another Testament of Jesus Christ, a record of God’s dealings with the inhabitants of ancient America (2000 BC – 400
- The Doctrine and Covenants, a collection of revelations and inspired declarations given for the establishment and regulation of
the Church of Jesus Christ in the last days (1830 AD – 1978 AD).
- The Pearl of Great Price, a selection of revelations, translations, and writings of Joseph Smith.
This demonstrates that the Bible is understood as fairly well recognized "collection" of writings, which is different from the "in addition to the Bible" collection of writings that are sacred to the LDS church. Here we have a list of three specific books that would be off topic directly for interpreting, as they are not part of the Bible, but may be allowed in a commentary fashion to support interpretation (as any other commentary would be allowed).
"Sacred" Text as Interpretation but Not Necessarily Text Being Interpreted
Sacred Text Inclusion
Those following Judaism recognize what I and other Christians refer to as the OT as the Bible, but do not recognized the NT as the canon of the Bible. However, they do recognize that Christians recognize the NT as such, and so the NT is on topic for asking about an interpretation of the text.
Christians recognize a relatively small subset of books as the Bible from both OT and NT, though that subset varies. Other Christians recognize that different traditions have a different Bible, and so those books are on topic for asking about an interpretation of those texts.
Mormons, as noted above, don't even classify their sacred books as the Bible. So they are off topic for asking about an interpretation of those texts.
Sacred Text as Commentary
Commentaries are not authoritative like Scripture, but they still can have argumentative weight. However, that weight is dependent upon its faithfulness to what the text of Scripture does say. So...
A Christian interpretation of an OT text can use a NT text for support (like commentary). One following Judaism can attempt to discredit that under their hermeneutics.
A Mormon interpretation of an OT or NT text can use a Mormon text for support (as commentary). One following Judaism or another branch of Christianity can attempt to discredit that under their hermeneutics.
Essentially any text can be used as commentary support (and commentary is all we are offering on this site), but all commentary may need to be further backed up by the Biblical text itself, or face possible defeat by the testimony of the Biblical text itself from another viewpoint (obviously the best defeaters use those texts recognized by all parties involved in the discussion).