Yet Dallas is still home to the longest light rail system in the world. The trains are not an issue, it's how decentralized the people are. The value is simply not there to justify the project in most people's eyes (and won't benefit most people).
https://en.wikipedia.org/wiki/DART_Light_Rail -> System Length 93 mi (150 km) https://en.wikipedia.org/wiki/Trams_in_Melbourne -> System Length 160 miles (250 kilometres)
However, your point that the decentralization plays a difference is very valid. The Melbourne network is a dense grid which serves a much more compact area whereas the Dallas system consists of a small number of lines that stretch very far out into the low-density suburbs. Hence, far fewer people are near a DART station.
0 - https://en.m.wikipedia.org/wiki/Light_rail vs https://en.m.wikipedia.org/wiki/Tram
Edit: another example of Wikipedia not being able to agree with itself on a distinction -- https://en.m.wikipedia.org/wiki/Light_rail_in_North_America lists the Toronto streetcar system as "the busiest LRT system in North America" even though it's definitely a traditional tram system with mostly street-running. Below it calls it a light rail ('in the case of Toronto, actually streetcar').