Whywhy did the European population grow rapidly in the 1800s

Mostly because the death rate fell tremendously.

Explanation:

The main causes of population growth had to do with diet and hygiene and health care.

The Agricultural Revolution (which started in the 17th century) led to better food production and thus better diet and nutrition for people, so healthier lives.

The Industrial Revolution (beginning in the mid 18th century) pushed cities to create better sanitation methods to avoid cholera and other disease epidemics.

And the ongoing advances in medicine ever since the Scientific Revolution were doing their part also to increase lifespans.