If any were able to escape while it was moving that would imply the door was ajar or some other opening was made and it would still be open when it stopped. So how did it get closed again, assuming there were still people trying to get out? How would the driver or accomplice push it closed again with presumably panicked people trying to stream out of there like a herd of cattle? Maybe by holding a gun on them to frighten them to stay? Is it possible the ones found on the roadside were ones who died on the trip and were just pushed out along the way by fellow migrants - or cartel guards- to make more space, rather than being escape attempts, that the exit was always in the control of the smugglers?
Or maybe they got out when it stopped to walk away but we’re too weak to get far and died on the roadside.
The powers that be are not covering it or not bringing it up because they don’t want stories like this to scare immigrants into staying home.