I am not sure about the CAA requiring reversers as suggested by GEnxsus. I attended a number of the early A3XX working groups (before it became the A380) for my then employer in the late 90's and the reverser issue was raised by Airbus.
As others stated Airbus preferred the no reverser option to save weight and complexity and maintenance costs plus there were so many wheels that could be braked however the airline working group recommended Airbus go with the inboard reverser option. The reason was, if I recall correctly, in one scenario put forward by Airbus the required runway length was very long thus possibly preventing a max gross weight TO on a wet/slippery runway at some airports.