Monorails are somewhat cheaper than elevated trains (if you don't have extreme disabled-evacuation laws) but more expensive than a conventional train at ground level, and junctions are a nightmare. So they only work where you have a single isolated line and would need to elevate most of the track anyway (and suspended rather than straddle-beam improves cornering performance but at a cost, so is only worth it if your route has many sharp corners as well).

For Wuppertal, where the town is pretty linear along a river valley, it works. (Even then, a straddle-beam monorail would probably be more cost-efficient if you were starting from scratch). For most places it doesn't.