Light rail densifies corridors, costs less when used more, increases property values, and is generally significantly faster.
You also have to consider the demographics/economies of Phoenix, and Las Vegas. Vegas is a tourist city with a significant number of people living near or below the poverty line, Phoenix has its fair share of poverty as well but more people living there are in the middle-upper class, meaning they are fare more likely to drive. Phoenix also has less density in its core, so it makes sense that ridership is a bit worse there than in Las Vegas.
The ridership of Phoenix isn't "a bit worse" than Las Vegas, it's a lot worse. Las Vegas has 66 million transit boardings annually and a population of 2.4 million while Phoenix has 71 million boardings and a population of 4.8 million. Las Vegas is one of the leaders in per capita transit ridership in the US, and they do it with buses only. I am not sure how much demographics are a factor when talking about transit ridership between different entire metropolitan areas.
It's only bus-based systems like Las Vegas, Seattle, Austin, Hartford, and Pittsburgh that are seeign ridership growth, while all of the rail-based systems in the US are losing ridership. The US systems made a mistake assuming rail is more attractive than bus. Rail follows ridership, not the other way around.
People should look at the ridership of the US systems that have at least 60km of light rail or subway and compare them to Mississauga:
MARTA (Atlanta): 115 million boardings
RTD (Denver): 98 million
Trimet (Portland): 97 million
MTA (Baltimore): 95 million
San Diego MTS: 85 million
Dallas Area Rapid Transit: 60 million
MiWay (Mississauga): 55 million
UTA (Salt Lake City): 44 million
Metrolink (St. Louis): 37 million
Sacramento RTD: 21 million
MiWay's ridership is forecast to grow by 15% in the next 4 years to around 63 million boardings, surpassing DART (149.7 km of light rail) and approaching the San Diego MTS (86.1 km of light rail). The time to build Dundas BRT was 30 years ago and I think that time is past. It's not a minor corridor for some small transit system anymore. This BRT that Metrolinx is pushing is all about Halton's needs, nothing to do with what Mississauga needs.