The life expectancy in the united states increased more than 30% from 1930 to 2015. which factor could have contributed to this increase?

This would mainly come from the medical information that we now have. Back then, smoking (for example) was normal and definitely not considered unhealthy. Now we know that smoking is terrible for you.
Along with that, we're closer to solver cancer than ever, there is more help for suicidal people, and technology is advancing so quickly to help us solve medical issues.
I would definitely say that the main factors for this increase are our newfound knowledge and technology helping us along the way.