Life expectancies in the united states rose dramatically in the past century due to

That's a vey interesting question.  Why has "life expectancy"
risen substantially in the US in the past Century ?  Another way to
put it ... "Why has the average age of everybody gained so much ?". 
Or  "Why is there a greater part of the population living to more
advanced ages now, like into their 70s, 80s, and 90s ?"

I think most of us have the impression that the greatest advances
have been in treating the ailments and common problems of old
people, like arthritis, heart disease, cancer, general fitness and
nutrition etc.  There has certainly been a lot of progress in those
areas, at the high end of the age distribution.

But . . .

It turns out that an even greater improvement in the statistics of
our longevity as a nation has resulted from great accomplishments
at the other end of the age spread ... the reduction in infant mortality,
and the conquest of diseases that killed great numbers of children
in the early 20th Century:  Measles, mumps, whooping cough, flu,
chicken pox, diphtheria, rheumatic fever, even polio.  Each of these
was a dreaded killer of children 100 years ago.  By the time I was
a kid, they were rites of passage ... everybody gottum, got better,
and returned to school.  And now, you hardly ever hear about
somebody getting any of these, and if there are a few cases in
the same year and in the same city, it's a big deal.  They've been
largely wiped out.  So there are far fewer childhood deaths than
there used to be, puling down the nationwide average lifespan.