It's not that the FAs can't lift 10kg, but simply that this is the acceptable risk threshold the airline has chosen. Anyone who lifts things repeatedly as part of their job stands a greater chance of being injured by heavier items - especially if awkward postures (lifting over your head, trying to horizontally maneuver the bag into an overhead bin) are involved.

In an emergency, I'd imagine such rules can be waived when the goal is to e.g., get all the passengers off a burning plane quickly.