That this was an almost unique freak incident with a truck dislodging a manhole cover, explains why manholes covers are not bolted down.
It is like a 1 in a million or 1 in 10 million chance of happening. It is almost unheard of for a manhole cover that is correctly fit and in place to just get bounced out of its supports.
I guess bolting them down woujld prevent that 1 in a million tragedy like this lady, but it does validly explain why they aren’t bolted down. There is typically no need. Typically.
Maybe there is something wrong with this manhole rim or cover. And that made it susceptible to this fault.