The Ramban is of the opinion that the world's air quality was altered after the Flood, and other explanations are advanced for the shortening of the human lifespan. I would like to address the underlying pattern and system for this and other changes of nature in antiquity.
It is well-known that Man originally existed on a highly-elevated plane. Not only was he to be sustained by Hashem without much human effort and could he procreate instantly (see Rashi from Medrash on 4:1), indicative of a superhuman existence. The truth is that Man was of a different caliber altogether, such that he conversed with God as did Moshe (as we see in the incidents of Adam HaRishon and Kayin) and was more of an angelic being. Targum Yonasan ben Uziel on 6:3 explains that God instilled His "Ruach Kodesh" (Holy Spirit) in Man in order for Man to do good. It must be noted that this Ruach Kodesh refers to the neshama, the holy, divine spark component of the soul, and not the nefesh, which is the mere force of life. Thus, Man operated with a highly-elevated spirit within him, and it appears that this spirit was not renewed in all people after the Mabul.
The Targum on verse 4 (ibid.) concurs with interpretations that the Nephilim were angels who were expelled from Shomayim to the earth, and they seemed to have functioned within society for some time, as the Targum implies in its continuation of the narrative. Again, we derive that the contrast between Man and celestial beings was not so pronounced.
Parshas Noach changes things. Man was permitted to eat meat, he lived for much shorter periods, and the seasons of nature were now fixed and predictable. This metamorphosis of Man's existence in his world reflects a secularization and downward thrust in the position of Man and his surroundings. A long lifespan is more similar to an eternal, celestial one; shortened life represents the temporal state of Man.
After the Dor Haflogoh (Generation of the Dispersion) who built the Tower of Bavel, life became even shorter. Average longevity decreased from 500 or so years to less than half that. (See chapter 11 of Bereshis.) So, too, the Holy Tongue was no longer standard, and secular languages were spoken in the majority. This further indicates a lowering and secularization of Man's state. (Rashi on 11:1 quotes Chazal who indicate that the Dor Haflogoh objected to Hashem's exclusive and total mastery over the Heavenly beings. This may be interpreted as emphasizing that the true aim of the people was to rebel in the pursuit of restoring Man's former lofty spiritual status. This notion may be supported further by the Targum Yonasan ben Uziel (ibid.), who explains that the masses spoke "the Holy Tongue, with which the world was created..." This accentuates the desire of Man to prevent complete disconnection from his old stature and and remain a quasi-Heavenly force on his own, as it were. Obviously, Man's intentions were counter to the Divine plan and his actions were offensive and belligerent, rather than supplicative and appeasing.)
The Hebrew root-phrase "chal" or "chol" (ches-lamed) is astonishingly utilized about a dozen times in Parshas Noach, with different meanings and contexts. The term means "secular" or "unholy", and its coded use underlies the transformations of nature and Man which form the theme of Parshas Noach.
However, there is still hope for Man to attain his former, elevated state. The piyyut "Amitz koach" recited on Yom Kippur compares Bnei Yisroel's existence to that of Original Man. (See Ramchal in Derech Hashem for a full treatment of this comparison.) God wished for Man to be able to attain his holy tasks in this world, and He renewed this ability with the birth of Am Yisroel. The Maharal explains that only Bnei Yisroel bear the neshama referred to above. In fact, one may compare our golus (exile) to the expulsion of Man from Gan Eden, such that communion with God and open miracles are precluded from our state of life at present, and toil, suffering and hester panim (God's Hiding of His Presence) are the norm, similar to the existence of Man after his initial punishment. However, we are told by the Torah that this state of affairs is temporary.
May we merit to be restored to our initial, pristine relationship with Hashem, and draw ever closer to Him.