Actually, it should be the category entitled "The Bible" that is divided that way. While the Bible is a very important book for Christianity, putting all the books of the Bible under the "Christianity" category doesn't make nearly as much sense.
Another answer
This may become possible in the future. When they become large topics we can subdivide them. Also... Many scriptures touch on multiple doctrines, and can be used for very different reasons, so dividing a scripture into one section, when it can be used in many, will cause massive repeats of scripture, and as it all ties together, you will have one big mess.
I would suggest you find teachings on doctrine, and let them lead you through different points of the bible in reference to the doctrine, instead of trying to divide the bible up into sections.
Another answer
The reason the bible is not printed in category format is because of what Christ stated in Revelation 22:18-19, therefore no man or woman dare change the written word.
There are many works which categorize the bible into topics which one can search There are other works which allow the biblical texts to be searched by word. Its always best however to read the entire text of a chapter, in order to follow the thought being presented (subject and object) simply searching for snippets here and there, will provide little meaning and is of little use, related to actually learning the texts. Missing the complete thought can cause great misunderstanding of what is written, if only snippets of text (topics) are relied upon. Christ teaches on three levels (beginner, intermediate and advanced) at one time, the reader can loose the topic not understanding this point.
The book of Psalm is subdivided into five smaller books.
... The Bible
One, the "Bible".
The Holy Bible
The scriptures of Christianity are contained in the Bible: 39 books of the Old Testament and 27 books of the New Testament.
You need to treat a Bible Book with care. The Bible is God's Word, so we should all treat Bible's and books that talk about the Bible and Christianity with care.
books and stories of saints study guides to help understand the bible. . .
A Mere Christianity, by CS Lewis. Imitation Of Christ, by Thomas A. Kempis.
Globally speaking, the Holy Bible.
The Torah is the First 5 Books of the Holy Bible.
The Holy Bible is from God and is the scripture of Christianity.
The Jews call these books The Tanakh (Jewish Bible), while the Christian name is the old Testament.