Monorails are usually not the most practical solution. In most places where you can build a monorail, a regular train is usually simpler and cheaper to build. Wuppertal has some very specific geography which causes a monorail following the path of the river Wupper to make sense, but that often doesn't translate well to other places.

Tom Scott made a video about it at some point:
https://www.youtube.com/watch?v=F4KZLcvMQWg

And a video on the Schwebebahn by The Tim Traveller:
https://www.youtube.com/watch?v=9IFh6wFTJiQ

The situations where such infrastructure technologies make sense as the best option are pretty rare. Wuppertal had the unique combination of circumstances with:

• a decently well-connected inventor and a supportive emperor
• the urban/metropolitan area being in a narrow valley along the river Wupper
• steep hillsides poorly suited for rail
• geology poorly suited for tunnelling with late 1800's technology
• local steel industry
• the ability to use the river as the right of way
• the relative infancy of electric railway propulsion

This all contributed to a suspended railway making a lot of sense in that case.

Today, an average problem can be satisfactorily addressed with a form of conventional rail in most situations.