Easy ... they got lazy or distracted or had a miscommunication.
I can't speak from a piloting perspective, but in my own line of work I've seen any number of failures and mistakes caused by failure to perform some basic check. I've made a few such mistakes myself.
As I said, though, it's just a guess. There are other things that could cause a plane's engines to shut down as described ... but fuel seemed to me the most obvious candidate.
Speaking from my perspective, that is exactly what all of our checklists are for.
There are procedures in aviation, and using a checklist is the biggest and most important one.
Paramount.
I cannot see how pumping the sumps to check the fuel could possibly not be done, especially when there is an instructor flying right seat.
It is right on every checklist ever made.
Your life depends on these rules every time that you are wheels up.