Because outside the English-speaking world, people have been using various encodings that predate Unicode and are tailored for their respective languages for decades. These language-specific encodings have become ingrained everywhere and are pretty much a standard. If you want to have any hope of interfacing with legacy systems, you have to use them, so all systems have to support them and usually use them as default even if they by now support UTF-8 as well. There may even be multiple legacy encodings traditionally used for different purposes.
Examples:
- ISO-8859-1 in western Europe - actually outdated there as well, as you need ISO-8859-15 for the Euro sign
- ISO-2022-JP in Japan for emails, Shift JIS for websites
- Big5 in Taiwan
- GB2312 in China
The last two examples show that encodings can even be a political issue.
