Here is how \ufffdThe Chicago Manual of Style\ufffd (14th ed., pp. 160-61) describes the rationale behind its position:

Periods with Quotation Marks

AMERICAN STYLE

5.11

When a declarative or an imperative sentence is enclosed in quotation marks, the period ending the sentence is, in what may be called the American style, placed inside the closing quotation mark. If the quoted sentence is included within another sentence, its terminal period is omitted or replaced by a comma, as required, unless it comes at the end of the including sentence. In the latter case, a single period serves both sentences and is placed inside the closing quotation mark.

\ufffdThere is no reason to inform the president.\ufffd

\ufffdIt won\ufffdt be necessary to inform the president, \ufffd said Emerson.

Emerson replied nervously, \ufffdThe president doesn\ufffdt wish to be informed about such things.\ufffd

BRITISH VERSUS AMERICAN STYLE

5.13

The British style of positioning periods and commas in relation to the closing quotation mark is based on the same logic that in the American system governs the placement of question marks and exclamation points; if they belong to the quoted material, they are placed within the closing quotation mark; if they belong to the including sentence as a whole, they are placed after the quotation mark. The British style is strongly advocated by some American language experts. In defense of nearly a century and a half of the American style, however, it may be said that it seems to have been working fairly well and has not resulted in serious miscommunication. Whereas there clearly is some risk with question marks and exclamation points, there seems little likelihood that readers will be misled concerning the period or comma. There may be some risk in such specialized material as textual criticism, but in that case author and editors may take care to avoid the danger by alternative phrasing or by employing, in this exacting field, the exacting British system. In linguistic and philosophical works, specialized terms are regularly punctuated the British way, along with the use of single quotation marks. With these qualifications, the University of Chicago Press continues to recommend the American style for periods and commas.