It has a few compiler extensions that are very useful for small embedded development, but GCC won't add (eg an "at" keyword to let you place a symbol at a specific address).

As for "why" nobody has done it, it's probably just because nobody has. The targets are mostly 8-bit devices with limited resources and SDCC is good enough. Typically the vendors have their own paid toolchains. I don't really know how well GCC would perform on those targets (although AVR8 uses GCC).