Italics get used to express important semantic meaning, so unicode should support them

[David Starner]

There's a lot of good answers, but I'd like to circle back to what I
think is the core reason: we've had character sets for seven decades,
virtually all of which supported English, and if any have supported
italics, I've never heard of it. Unicode supports italics the most of
any character set I've heard of. Whether in some sense italics should
be encoded in plain text is not an open problem; it's been assigned to
a level above plain text, and is well supported there.

-- 
The standard is written in English . If you have trouble understanding
a particular section, read it again and again and again . . . Sit up
straight. Eat your vegetables. Do not mumble. -- _Pascal_, ISO 7185
(1991)