The same reason anyone loses interest in anything. They either become disillusioned by the subject, or distracted by another.
they are a way to support and potentiate tradition. also they standardize the particular method.
The Islamic Empire was expected to last at the peak of its power for at least half a millennium. It lasted for more than half a millennium from 1299 all the way to the end of World War I in 1918....
to earn money
All Christians believe in God. Someone would be a Christian in name only if they didn't - which means they aren't one at all.