Q. Why are prepositions (and other such words) lowercase in titles in Chicago style (per CMOS 8.159)?
A. Except as the first or last word in a title, prepositions (and a few other categories) remain lowercase in titles in Chicago style—and in most other styles—because they’re not considered important enough to capitalize.
The first edition of the Manual (published in 1906) said to capitalize “all the principal words (i.e., nouns, pronouns, adjectives, adverbs, verbs, first and last words) in English titles of publications” (see ¶ 37). In the examples of titles that followed this advice, only the, and, of, and on—one article, one conjunction, and two prepositions, all of them short—were lowercase.
This advice remained mostly unchanged until the twelfth edition (1969), the first one to name the lowercase exceptions—and to clarify that length isn’t a factor: “Lowercase articles, coordinate conjunctions, and prepositions, regardless of length” (7.123; emphasis added).
A new paragraph demonstrated the rule and included a title with a very long preposition: “Digression concerning Madness” (12th ed., 7.128; emphasis added). Many other styles, including AP, APA, and AMA, would capitalize “concerning” in a title simply because it’s long (four letters is a common limit for prepositions).
At least one style guide, New Hart’s Rules (2nd ed., Oxford, 2014), takes a less strict approach—for example, recommending lowercase for possessive pronouns and allowing for exceptions based on appearance: An Actor and his Time and All About Eve (“a short title may look best with capitals on words that might be left lower case in a longer title” [8.2.3]).
So the rules have always been a little bit arbitrary. But Chicago and most other styles include guidelines that are designed to help writers and editors make quick decisions rather than considering the words in a title on a case-by-case basis. For more on this subject, see “Is ‘Is’ Always Capitalized in Titles?” at CMOS Shop Talk.