Improvements in car seats would be my guess. Plus, vehicles were required by law to include special latches for anchoring car seats starting around the early 2000's. It was surprisingly tricky to properly install a car seat before those latches.
Figure 10-20 years for old cars to get swapped out for new ones and we're probably at a point now where most cars are equipped with those latches.
Figure 10-20 years for old cars to get swapped out for new ones and we're probably at a point now where most cars are equipped with those latches.