Some of the eye catching special effects visuals in the Star Wars franchise include an incredible number of small flying personal vehicles traveling through cities bumper to bumper to who knows where at high rates of speed. Do any of the city scenes ever depict inner city mass transit, or are all the vehicles either personal or freight carriers?
1. George Lucas, Car Aficionado
This paragraph from Lucas' wikipedia biography seems interesting, outlining the central role cars have had in George Lucas's life:
[H]is early passion for cars and motor racing would eventually serve as inspiration for his USC student film 1:42.08, as well as his Oscar-nominated low-budget phenomenon, American Graffiti. Long before Lucas became obsessed with film making, he wanted to be a race-car driver, and he spent most of his high school years racing on the underground circuit at fairgrounds and hanging out at garages. [...] He also began filming with an 8 mm camera, including filming car races.
I have a feeling the Pod Race from Episode 1 was a scene he had wanted to film for most of his life - just like seeing the city sky filled with cars of all shapes, sizes and vectors.
2. George Lucas, product of the American Commuter Culture
However, I would like to add another, more speculative reason:
Sometimes science fiction is used as a thought experiment, a social laboratory to view the effects of science or technology on society. The effects of planetary-scale urbanization is a fascinating topic of discussion, and its effects on transportation only one facet.
Star Wars is, on the whole, not that kind of science fiction.
Star Wars uses these futuristic cities more as scenery than as the focus of discussion. It adds to the alienness, but is not what the story is about. And that's perfectly fine, yes? Not everything has to be about the effects of urbanization, even if it is one of my favorite topics. Star Wars is not about that.
So, seeing that George Lucas' didn't go about creating a coherent future-city, it seems to me to make sense that he simply took the world around him, the California car and commute culture, and transplanted it into his future alien worlds.
Had George Lucas been born in a bigger city where mass transit was a bigger part of life, this might have been different.
3. Only The Rich Reach For The Sky
Donald McLean brings up another excellent point: Coruscant is a completely urbanized, built-up world. A huge complex of buildings spanning the entire planet. In this case, why would we need flying buses or space-trains, when an internal transportation system like a branching network of elevators - can do the trick?
Coruscant is known as an homage to Isaac Asimov's Trantor, another capital of a galactic empire that's entirely covered by a huge city. In his Foundation, from 1951, Asimov notes that the inhabitants of Trantor, so used to living indoors, never go outside even if they can, shunning even the observatories and lookout spots. Drawing from that, we can speculate that on Coruscant, most people don't travel by sky-bus, or even on a personal jet, because most people never see the outside world. Anakin, as a respected Jedi, has an extremely fancy apartment with not only an outside view, but an actual open balcony (with absolutely no handrails, of course). I think the personal transports we see in the sky are pleasure craft, luxury items for those rich enough to see the sky.
I know that mass transit is seen in Episode II. But it isn't on Coruscant that this scene is filmed. However, given the fact that it is seen in one episode lends credence to the fact that we should expect it to be on a Core World.
And I am fairly sure I have read about the use of mass transit in one or two passages in a novel. I couldn't begin to guess at the title though.
One of the planets in the MMORPG Star Wars: The Old Rebublic has a monorail system for players use. Up to you whether you class it as cannon.