“Why” is a question for Microsoft, but I'm guessing it boils down to a simple lack of resources on the part of the Mac Business Unit. They have to prioritize certain features, and RTL support is not a priority for them.
To address the underlying need, you have several options:
OpenOffice and NeoOffice support RTL text in Microsoft Office documents.
Mellel has a reputation as the best RTL/multilingual word processor (it certainly has a nice feature set for it, like a direction breaking space so you can mix RTL/LTR in a paragraph), but I don’t know how good its Microsoft Office document interoperability is.
This would also be a use case where it might make sense to run Office 2010 inside a Windows virtual machine.