[We have those now. They are pretty much known as TRAINS!]
There aren’t enough rail tracks to move all the goods that need to be moved. Maybe when the population was a fraction of what it is today. And you have to get the container from the track to its final destination.
[As a side note: Indianapolis has shut down it’s driverless monorail.]
The issue is probably more the non-economic nature of the monorail. There are city bus systems where only 10% of the cost is recovered through fares. Monorail systems are even less economic.