Origins of italic type and its shrunken role
Italic type originally served very different roles. It wasn't invented just to set off words that were emphasized or foreign.
The humanist spirit driving the Renaissance produced its own unique style of formal writing, known as "cursiva humanistica". This slanted and rapidly written letter, evolved from humanistic minuscule and the remaining Gothic current cursive hands in Italy, served as the model for cursive or italic typefaces....Origins of the kana syllabaries
Surviving examples of 16th century Italian books indicate the bulk of them were printed with italic types. By mid-century the popularity of italic types for sustained text setting began to decline until they were used only for in-line citations, block quotes, preliminary text, emphasis, and abbreviations.
In A History of Writing in Japan (U. Hawai‘i Press, 2000), Christopher Seeley describes the origins of the kana syllabaries (p. 59).
The two Japanese syllabaries known to us today as hiragana and katakana came into being as the result of a process of simplification to Chinese characters used as phonograms [purely for sound, not meaning]. The phonogram principle was known in early China, where it was sometimes utilised to represent foreign words in writing, as for example Sanskrit names and terms in Chinese translations of the Buddhist sutras. In Japan, too, Chinese characters were employed in this way from an early date, at first only to represent proper nouns, but subsequently in an increasingly extensive manner. This gradual trend towards the wider use of phonograms provided a strong incentive to the development of simplified forms.Hiragana developed through a process of cursivization—linking, blurring, and eliding separate strokes in order to write whole characters more rapidly (a bit like cursive script and its derivative italic type in Europe). Katakana developed through a process of writing just one key part of a whole character.
Early roles of the two syllabaries
Nowadays, hiragana is the more basic of the two syllabaries, in that respect more akin to roman type; while katakana is used to represent foreign words and names, onomatopoeic sounds, or emphasized words, in those respects more akin to italic type. However, the earliest common usage of katakana was to gloss Chinese characters with their native Japanese translation in kuntenbon, Chinese texts marked for reading as Japanese, dating from around the tenth century. In those glosses, katakana indicated the native Japanese reading (kunyomi), not the foreign reading (onyomi for Sino-Japanese). This style of reading Chinese texts, called kundoku, required the reader to translate each Chinese sentence not just into native Japanese word order, but into native Japanese words, even adding Japanese honorifics. Readers in the ondoku style, by contrast, would render the Chinese text in Chinese order and Sino-Japanese (onyomi) pronunciations.
While monks and learned gentlemen decoded Chinese texts with the aid of katakana glosses, noble court ladies employed the more elegant and flowing hiragana to compose Japanese-style letters, poems, and prose fiction. In fact, cursive hiragana was referred to in those days as onna-de 'women's hand' (the term hiragana is not attested until 1603); while otoko-de 'men's hand' denoted a blockier script heavily dependent on Chinese characters (Seeley, pp. 76-80). This doesn't mean that men never wrote in hiragana, or that women never employed kanji or katakana, only that cursive hiragana was considered more feminine, and blockier kanji and katakana was considered more masculine.
Kata the mechanical kana
As Japan opened up and began industrializing in the mid 1800s, the relative simplicity and efficiency of katakana gained it many new applications, most notably in semaphore, where the flag positions represent the shapes of katakana strokes (requiring 1, 2, or 3 positions per character); and in telegraphy, where Japan's Wabun kana-based Morse code was far more efficient than China's character-based code, even though it requires twice as many dot-dash combinations as Oubun 'European' Morse code. The two superscript dots in Japanese kana that indicate voicing (dakuten) are efficiently coded by an extra dot-dot, but the superscript circle that turned h into p (handakuten) is coded far less efficiently by an extra dot-dot-dash-dash-dot! In both semaphore and telegraphy, the receiver transcribed the message in katakana and telegrams were delivered in katakana.
As a result, military communications were overwhelmingly rendered in katakana. Bill Gordon's very impressive website, Kamikaze Images, even includes a replica of a kamikaze pilot's final letter to his children written almost entirely in katakana. And former RAAF wireless operator A. Jack Brown, who spent World War II transcribing Japanese military broadcasts, even titled his recently published memoir Katakana Man.
Instead of a flying career, Jack found himself in top secret RAAF wireless units. There he worked to intercept radio transmissions sent in the Japanese katakana code, which were then analysed to produce the highly reliable intelligence that helped General MacArthur in devising his strategy for the allied campaign in the South-West Pacific.(Also see the U.S. Naval War College Review article about American code-breakers in the Pacific.)
In some ways, katakana also played a role similar to that of the Courier typeface that was the official standard for U.S. government and diplomatic documents for decades until 1 February 2004. Government reports were often published in kanji and katakana, rather than kanji and hiragana as would be customary today. So was Japanese imperial propaganda (translated here). Ease of carving also made katakana much more common in official seals and on woodblock prints than it is today.
I suspect the wholesale abandonment after Japan's defeat of so much katakana usage was partly motivated by Japan's attempt to wash away the stains of its military and imperial legacy.