Multiple Inheritance in XML Schema

Gurus,

Why isn't there a straightforward mechanism for multiple inheritance
in XML Schemas?
What are the rationales for that design decision? Is it because it
adds unnecessary complexity?
I am sure there are strong reasons for this - am interested in knowing them.
Ram

-- 
Shift to the left, shift to the right!
Pop up, push down, byte, byte, byte!

-Ramkumar Menon
 A typical Macroprocessor