Shortwave radio can be transmitted around the curve of Earth by ionospheric reflection and refraction so fewer repeaters are needed. This allows crossing vast oceans where microwave infrastructure might not be possible.
As you say the downside is available bandwidth and throughput.
In theory having fewer repeaters improves latency, probably in the range of 100ns per repeater. I don't know how much of a practical effect that has, likely very minimal with modern implementations.
Either way it's more sensible to build high throughput microwave networks given the tiny amount of shortwave bandwidth we have.
Most markets, in terms of their daily volume, are open at night, but very thinly traded until EU hours, but some do see action in Asia hours. It’s just about liquidity.
Maybe you’re thinking of single name equity markets, which are a fraction of daily trading.
I think they’re probably asking about US markets. Afaik those do close at business hours and I’m not sure how after hours trading happens but it might not be available to most people.
I think the one of the main reasons is government concerns about shenanigans happen overnight without oversight / flash crash. That being said I presume all the breakers that would halt trading activity are probably automatic but potentially not all of them (I think the US did things like that during the housing collapse).
As you say the downside is available bandwidth and throughput.