This is similar to the issue of Australia's national anthem. Our former national anthem was God Save the Queen. A national anthem also needs to be instantly recognisable and because this anthem was also used by the UK the change to another anthem was made. This was mainly because it was too confusing at international events, such as the Olympics.
Our present flag also features symbols that are not unique to Australia and do not set us apart from other countries. The Southern Cross, for example, is associated with many other countries in the southern hemisphere, not just Australia. The Southern Cross appears on the national flags of Papua New Guinea, Western Samoa, Solomon Islands, New Zealand and