Why haven't they been popular in Australia before?

Most fifth wheels manufactured in the US are 30 feet long or more and require relatively large vehicles to tow them, e.g Ford F250 or F350. In the US vehicles such as Ford F250s are very popular, mass produced and relatively inexpensive. They are as common as utes in Australia. The F250 and F350 have only been available from Ford Australia in recent years.

Fifth wheels suitable for Australian utes have only just become available partially as a result of the awareness of the larger ones coming into Australia.