How about because it's stated in the community rules?
"Formatting: Please post in plain text. The occasional bold or italic tag is fine, but extensively formatted entries won't properly display on many people's friends pages. Please do not post using large or colored fonts."
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Date: 2011-04-14 06:29 pm (UTC)"Formatting: Please post in plain text. The occasional bold or italic tag is fine, but extensively formatted entries won't properly display on many people's friends pages. Please do not post using large or colored fonts."