Why do you think life expectancy for men and women in frankish times was so much lower than life expectancies today?

The main reasons for this are better health care, better nutrition, and less physically stressful lives.

In the Middle Ages, medical care was pretty woeful.  There were, of course, no antibiotics so any wound could become infected and potentially lead to death.  As mentioned in your previous question, childbirth was very dangerous.

People in the Middle Ages, and especially the peasants, were extrememly poor compared to people today.  They lacked proper nutrition, especially during the winter.  Even the rich didn't know about proper nutrition and so ate quite unhealthily.

Finally, the work done by people in these times was very physically demanding.  They could easily be injured (and lacked medical care to make them better).  Even barring injury, they would wear themselves down through hard work combined with poor nutrition.