In Egypt it was a minority of Jews who voluntarily left Egypt.
During the times of the Bais Hamikdosh, there were a large number of Jews who became Misyavnim.
In the 1900’s the Reform movement began.
Before the start of the Holocaust young Jews left Yiddishkeit in the droves for all type of “isms” -Communism, Socialism, Zionism.
There were always Jews who left Yiddishkeit throughout the centuries as this is BECHIRA- the ability to choose our spiritual destinies any moment of our lives.
Since Hashem created Man with the ability to choose his spiritual destiny, Man chooses. Because most humans do want to do good and not bad, but some still choose to go down the wrong path, excuses are needed for their devient behaviour. The same challenges that Hashem put before each human as a stepping stone to grow, can be claimed by some as THE CAUSE for making them slip down the slippery slope of sin.
Basically it boils down to bechira.