A little kabbalistic/deep, but refer to the Or HaChaim on Bereishis 47:29:
Originally, G'd assigned great tasks to the antediluvian generations and at the same time equipped them with commensurately greater souls, each one of which consisted of many "sparks." He assigned a single day to each "spark." This is alluded to in Exodus 16,4 when the Torah speaks about people collecting the manna on a daily basis, i.e. דבר יום ביומו. The word יום may be understood as ענף, a branch or sprout of each soul. G'd granted each human being a certain number of days, i.e. He allocated to them approximately 300.000 such gemstones as mentioned in the parable. After the failure which resulted in the deluge, all of this was restructured in order to help us achieve what is expected of us in a lifespan averaging seventy years.
The basic idea of this is that prior to the sin of man, the soul consisted of a lot of sparks. There was a lot of "work" to do in this world, and hence, as the Or HaChaim says, the lifespan was much bigger than after the flood.
Rabbi Dovid Rosenfeld wrote an article on this topic and explains that:
[...] when man became sinful, G-d began shortening his lifespan – in a gradual process which was not completed until the time of the Exodus. G-d did this so that man would be more strongly aware of his mortality. He would be forced to live his years knowing his soul would soon be returned to his Creator for judgment, and he would have to live with that awareness throughout his life.
This Chizkuni is also interesting, although it does not answer your question, it does deal with the subject of lifespan being shortened after the flood.