Because none of the parts that are needed for having a multilingual installer is present in the installation initrd. Don't forget that the installer is a very stripped down system that should fit in the few MBs of an initrd. There is no font that can support alphabets other than the latin one, you can't even display different characters in the installer, there is no gettext to parse translations (which also needs gcc and gcc-g++ libraries, which are also not present, so you can't just add it) and several other parts which I really don't remember right now.
Even if it was possible to add all those parts that are needed, without increasing the initrd size, the task of turning the huge collection of scripts that the installer is into a translatable system, is enormous and the installer would have to be completely rewritten in some parts. I won't do it. If someone else wants to give it a try, be my guest.