The big issue with monorails is primarily that they can't be easily combined with conventional rail. Subway trains and streetcars can be towed on the national rail system should the need arise. Not to mention that they can share rail and depots.
A secondary issue is that they aren't much cheaper to build than conventional rail on modern, elegant bridges. And the elevated monorails don't save that much air clutter compared to modern bridges for conventional rail.
If a city has an extensive network for subways and/or trams, there is absolutely no need to add another, incompatible rail type.
Monorails for freight won't happen anytime soon. Dresden, Germany employs it's tram network to carry goods to the automobile factory.