From Exploring Nature:
They cannot jump backward because of their thick, muscular tail. Their long feet and heavy tail also make walking impossible, forward or backward.
The Australian 'Coat of Arms' features an Emu and a Kangaroo:
Some say the kangaroo and emu were chosen to symbolise a nation moving forward. This is based on the common belief that neither animal can move backwards easily.
Can kangaroos walk backwards?
Is walking backwards just difficult for them or is it anatomically impossible?