Why don't we write "Biblical"?

I recently posted the following question to the Biblical Studies List:

What explains the growing tendency to spell the adjective Biblical with a small "b"? "Bible" is a proper noun isn't it? Only the Germans don't capitalize their adjectives.

I got the following answer:

You'll find that most publishers' instructions to authors require l.c. for all adjectives. Thus also deuteronomic rather than Deuteronomic. I think the only exceptions are adjectives derived from personal names, such as Josianic, Isaianic.

Here's my response:

This is interesting as there is no reason given here. According to English grammar adjectives are capitalized if they refer to Proper nouns, e.g. "English," "Bultmannian," "Jewish." The question is, is the "Bible" a proper noun? Here's a definition of proper noun:

A proper noun has two distinctive features: 1) it will name a specific [usually a one-of-a-kind] item, and 2) it will begin with a capital letter no matter where it occurs in a sentence.

The SBL handbook of style spells it with a capital: Bible. Not only that, it's logic on the rules of capitalization can only mean that Bible is a proper noun:

4.4.5. In general, a word or phrase used as a title of the whole or a specific part of the Bible is capitalized; the name of a genre is not capitalized. Thus any ancient and modern designation for the Bible, a book of the Bible, a division of the biblical canon (e.g., Pentateuch), or a discrete section of a biblical book (e.g., Primeval History) may be a proper noun and so capitalized.

What is the Bible (not bible) if not "a word ... used as a title of the whole"?

Here's an instructive example, again taken from SBL:

Psalms of Ascent is the name of a discrete subsection of the book of Psalms, but psalms of ascent is a genre of psalms like royal psalms.

In Biblical studies, the term "Bible" is not used to refer to a genre but to a particular collection of books. When we say "Bible" we know what it means without having to qualify it. The only context in which it is no longer a proper noun would be where it is just a genre designation, e.g. "The Silver Spoon is the bible of Italian cooking."

Writing "biblical" just feels ... I don't know ... disrespectful. Would you feel comfortable writing "bultmannian"? It somehow dissolves his identity into some larger category of phenomena.

Update: I'm delighted to see that I'm not the only one who thinks about such things. Kevi Edgecomb of biblicalia posted a related complaint three years ago in biblical or Biblical? He points out the logical inconsistency of the Chicago Manuel of Style, the role model for the SBL Handbook of Style.