1. Man was originally created to never die. Death, as you know, was introduced through sin, and was never part of God's created order or plan for mankind. Death is an intruder and God calls it elsewhere an "enemy." (1 Corinthians 15:26) So, since man was created to live forever, it doesn't seem strange at all to think that lifespans could have been nearly 1,000 years.
2. Obviously the gene pool has had a long time to get mixed and corrupted. Back before the Great Flood people could do things that we can't today without significant risk, such as marry a close family member. To do this in our day is to invite genetic difficulties, but when the gene pool was younger, people often married within their family units. So we know that from a purely genetic standpoint we are dealing with limitations today that didn't exist at an earlier time.
3. Scripturally we see lifespans began to decline sharply after the Great Flood. Why? Well...that's where we have to use some reason and even a little guesswork because, frankly, the Bible doesn't say. But BibleStudy.org has some interesting information on the subject:
The earth after the flood was dramatically different than it was before. The differences included the altering of the climate, atmospheric changes, changes in the hydrologic cycle, geologic features, a significant increase in harmful radiation reaching ground level, man's dietary habits went from solely plant-based to one where meat was eaten, and so on. These and many other factors led to man's much shorter lifespans.
The first century historian Josephus asserts that man, at one time, did live very long lifespans. He attributes their longevity to God's mercy and the fact that the food they ate was much better at extending their years on earth than it was after the [flood] (Antiquities of the Jews, Book 1, Chapter 3). (BibleStudy.org)
In keeping with what is mentioned above, I believe the Great Flood did a lot more than just wipe out all life from the earth so God could start over with Noah and his family. I believe it altered the landscape of the earth in such a way that the sun's radiation now has a much more profound effect on the aging process. But again, that's just my opinion. All we really know is that God allowed people to live much longer lifespans early on and then, for purposes He does not share with us, those lifespans were drastically reduced.