Still doesn't explain why they fail.
He listed the alternatives not to build one in the first place, but if one is built, why it fails?

He listed at least one reason, that because they are often single track systems, a single breakdown freezes the entire system.

Once it's assumed that it's not reliable or, worse, that there's a decent chance of being stuck in a suspended/elevated train until the whole system can be evacuated (requiring specialized rescue from every train that didn't stop in a station, then being 'down' until the breakdown is fixed or removed), people won't use it, and it will fail.

That doesn't explain why they don't just make all systems 2 track systems like the German one to solve that issue for a bit more money (but not double the money), but it does explain why some failed. It seems to me that they could still be a decent solution in some congested areas where light rail only adds to congestion, and they look neat.

A subway system is very expensive to build and they are all over the world. Also, subway lines are single lined, so when a failure occurs, the whole line, or a full section between two nodes is halted.

There is an interesting project in Quebec to develop a kind of intercity high-speed monorail system : http://www.trensquebec.qc.ca
it is said to be cheaper than a high speed train. It would use shuttles and be installed above the expressway.

We want more monorails!

Yes, but subway systems have spur tracks and track switches built in so a single broken train doesn't stop the line....usually. They aren't all really 'single lined' because you can often put a north bound train on the south bound track for a bit to get around a broken train/track/switch....not so on monorails.
Still, it seems like those problems can be solved if they wanted to. A triple tracked monorail would have been insanely cheaper and simpler than the Big Dig.