The Highway Forums General Discussion Open Forum Questions regarding the deuterocanon

This subject isn't exactly a well known one among most Protestants outside academia. But here is a link that will briefly define and describe what "Deuterocanonical books" are: Wikipedia article.

As far as why Protestants don't consider them to be canonical, the brief answer is that they don't meet the criteria of inspired Scripture. Perhaps Warfield's article on The Formation of the Canon of the New Testament will offer some help.

There are also some other possible relevant articles on the subject of "Inspiration" which can be found in This Section of The Highway.