As far as I'm aware, there isn't a "why" (at least not in the sense that it is a decision a bunch of people agreed upon); it's merely tradition dating back much further than digital fonts -- perhaps even back to the fact that italic and roman type developed individually, as separate styles of writing, and up until recently weren't even expected to combine in a single document or a single font family.
That said, there are hordes and hordes of people more experienced than me, so perhaps one of them has better information.
Posted Aug 13, 2012 2:00 UTC (Mon) by vonbrand (subscriber, #4458)
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I remember reading somewhere that the minuscule ("small one" in latin, called lowercase in English) was invented to save space on the page (parchment was expensive back then!), and that the italic style was also used in handwriting for the same reason.
Just a random, unreliable, faded memory.
Adobe ventures into open fonts
Posted Aug 13, 2012 17:11 UTC (Mon) by davelab6 (guest, #86237)
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The italic style was first used in small books, but the entire text was set in that style.
Today italics are meant to be visually distinctive and the lighter 'color' is one - important - way that type designers do this.
So I learned at the University of Reading's Typeface Design Masters programme :)
Adobe ventures into open fonts
Posted Jan 14, 2013 8:33 UTC (Mon) by pauldhunt (guest, #88795)
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What David said. Particularly in sanserif styles, it is more difficult to distinguish an italic meant for emphasis if the sole differentiation comes from the slope of the glyph forms. Making the italic somewhat lighter in color helps it to serve its purpose of differentiation from the upright style.