I would think that if it's such an important building the government would always try to keep it maintained. Where was the gap from when it was really important in antiquity to now when we think it's a valuable piece of history that should be preserved? And why that gap?
The sad thing is, it was pretty well maintained for the longest time. It was used as a temple (in the Greek period and under the Roman Empire), then as a church (under the Eastern Roman Empire), then as a mosque (under the Ottoman Empire). Each of those changes brought with it some removal of old things - mostly religious stuff from the 'wrong' previous religion - and some new additions, but overall the building remained intact.
During a war in 1687 the Ottomans fortified the hill and used the Parthenon to store gunpowder. When enemy artillery hit the Parthenon, the gunpowder exploded and the Parthenon was reduced to the ruin it is today.