What factors led to the decline of the Puritan religion in New England?

There were several factors that led to the decline of the Puritan religion in New England, but one of the most prominent factors was an increase in European immigration.

I believe the answer is: an increase in European immigration.

As European immigration increased, many people from wide variety of background start to come into New England and assimilate their culture with one another. At this time, many types religious of belief also come and found new followers, which lead to the decline of the puritans.