In Germany there is a lowercase letter ß. It actually is a ligature of the letters s and z. It does not have an uppercase variant, because there is no word that begins with it. One word would be Straße. If you write that all in uppercase, it technically becomes STRASZE, although you almost always see STRASSE. But if you write that all in lowercase without substituting SS with ß, you are making a mistake. And although Switzerland is a german-speaking country, they have different spelling and rarely use ß -- if not ever.

This is just one of many cases, where case-insensitiy would give more trouble than it's worth. And others pointed out similar cases with the Turkish language in this post.