The problem with conventional rail systems is that they don't have enough stations. They can serve lots of people if they're all in one place and they all want to go somewhere down the line, but most people want to go somewhere else, so they drive.
PRT guideways are cheaper, and the stations are cheaper and can be more closely spaced, so you can serve a much larger fraction of the trips people want to make.
For a longer explanation, see this link.
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The problem with conventional rail systems is that they don't have enough stations. They can serve lots of people if they're all in one place and they all want to go somewhere down the line, but most people want to go somewhere else, so they drive.
PRT guideways are cheaper, and the stations are cheaper and can be more closely spaced, so you can serve a much larger fraction of the trips people want to make.
For a longer explanation, see this link.