Because "Coca-Cola" is a "fanciful name", employing strange capitalization and hyphenation. It's also a household name world-wide, which allows protection of somewhat generic names.

It's a matter of common usage. Coca-Cola isn't a universal descriptor of that particular kind of carbonated soft drink. The term for that (in the US) is a cola (which is why there exists a lot of *-Cola beverages).

The Coca leaf isn't used anymore due to the prohibition of cocaine.