Reliability and lack thereof. Even if you take care of a car as it gets older it gets less reliable, things break more often, etc.
Amenities. As technology progresses, amenities that come standard in cars increase.
Value. This goes back to reliability and amenities. If you are having to buy something to fix the car every few weeks is it really worth keeping? You could be using that money to pay on a newer car with less miles and more amenities.