Why were Deuterocanonical books rejected in the Reformation?

Question

The Deuterocanonical books were included in the Septuagint, but not the Hebrew Bible. They are mostly included in the Catholic Old Testament, but not in the Protestant one.

I understand the choice was made by Luther, who called the deuterocanonical books

Apocrypha, that are books which are not considered equal to the Holy Scriptures, but are useful and good to read

Why weren't the deuterocanonical books considered equal to the Holy Scriptures?

Answer 1

A more ecumenical answer:

They weren't considered equal because they had been considered of dubious origin for quite some time. Back when the Vulgate was being put together Jerome made the points that

1. The original Hebrew for those texts could no longer be found*
2. Jews of the late first century onward did not consider them canonical.

Others in the Church countered:

1. The Church had been using the Greek copies for centuries (clearly evidenced in the New Testament and through the Apostolic Fathers)
2. Since the Septuagint included those books and the Septuagint was popular among the Apostles, it follows that these books should be on equal footing — whether the Hebrew copies existed or not.

The problem was that there had never really been an official ruling — the prevailing mindset was inclusion (especially since the Patriarchs all seemed to agree to their worthiness), but because the books were never challenged there had never been a need to define their proper place in the canon to begin with. When the early Protestants came along, their priority to get back to an earlier idea of the Church necessarily caused these texts to come into question.⁺ You can read more about that on Wikipedia.

It was not about Luther

There are some who have said that this is a decision which was made because Luther did not feel that the were consistent with his view of the Gospel, and there is a modicum of truth in that — he most certainly did not view them as entirely consistent with his theology — but that was not his justification or even his primary impetus. It is best to be wary of the stories about Luther in this context — many of them are apocryphal inserts by later apologists of one side or another.

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* It should be noted that the Hebrew of Sirach has since been found (and it is fascinating to compare the Greek and Hebrew texts, my Biblical Hebrew professor gave a wonderful lecture on it).
⁺ It should be noted that Luther was only one of the many early Protestants who included the Deuterocanonical texts but placed them at a diminished stature, and he certainly is not responsible for their exclusion from the modern Protestant Bible. I believe credit with their removal from the English Bibles was the 1611 Authorized Version (the third(?) edition of what we in the US call the King James Bible)