Why are tags typically all lower case?

Everywhere I look I see that whenever a site implement a tags system, they convert the tags names to lowercase. Even here in StackOverflow.

I was thinking about why is it so. Other than preventing duplication I can't think of a reason to use lowercase. I believe it hurts the practical aspect of the tags. People are used to read "IBM" not "ibm" and "C#" not "c#". It takes a bit more time for the user to understand whats the meaning of the tag, and I'm wondering if I should allow Capitals in my tags system, or is it a convention and I got it all wrong.

I want to hear your opinion.