We all know that people lived longer in different yugas than Kali yuga and I heard the life expansion decreased and now we can barely live for 100 years. So why did the life expansion decrease?
According to Vana Parva, Muni Markendeya explains this to Yudhishtira. The morality stands on 4 legs during Krita yuga, 3 legs in Treta yuga, 2 legs in DwApara yuga & 1 leg in Kali yuga. There seems to be direct relationship between Morality (read Dharma) and the age expectancy of men:
Yudhishthira said, ... O thou foremost of all speakers, O Muni of Bhrigu's race, that which we have heard from thee about the destruction and re-birth of all things at the end of the Yuga, is, indeed, full of wonder! I am filled with curiosity, however, in respect of what may happen in the Kali age. When morality and virtue will be at an end, what will remain there! What will be the prowess of men in that age, what their food, and what their amusements? What will be the period of life at the end of the Yuga?
This chapter discusses mainly about Kali yuga, as it has more extreme relation with Krita yuga. Here is the relevant part to the Qn:
Markandeya said, 'Listen, O monarch, to all that hath been seen and heard by me, and to all, O king of kings, that hath been known to me by intuition from the grace of the God of gods! O bull of the Bharata race, listen to me as I narrate the future history of the world during the sinful age. O bull of the Bharata race, in the Krita age, everything was free from deceit and guile and avarice and covetousness; and morality like a bull was among men, with all the four legs complete. In the Treta age sin took away one of these legs and morality had three legs. In the Dwapara, sin and morality are mixed half and half; and accordingly morality is said to have two legs only. In the dark age (of Kali), O thou best of the Bharata race, morality mixed with three parts of sin liveth by the side of men. Accordingly morality then is said to wait on men, with only a fourth part of itself remaining. Know, O Yudhishthira, that the period of life, the energy, intellect and the physical strength of men decrease in every Yuga!
Having said above, still there is no strong scripture basis which says that life expectancy was 1000s of years in earlier yuga-s. Ancient scriptures referred 100+/- years of life expectancy, which may vary with few decades, but not too many. Refer this and this answers. Some scripture translate sahastra strictly as "thousand". This might have created the confusion of 1000s of years IMO.