The religiosity of America is that by design or by a natural process?

The strong religious leanings in the USA probably relate to its history and the general high-mobility of people within the USA.

People, in general, like to feel part of a community and society but where there is high immigration - as there always has been in the USA - it is difficult for people to have a sense of "belonging". People don't have shared cultural backgrounds or experience in common. At the same time, people within the USA have been very mobile, they don't have roots in a community in the same way as people who live for many years in a comparatively small area, nor do they have family ties to an area.

For these types of mobile and loosely-linked communities, a church provides a centre for community and societal bonding. Churches promote cohesion and a feeling of belonging that people otherwise lack.

As societies mature, that is, as they develop their own culture, the role of churches in providing the comfort of "belonging" becomes less important. With the lessening of the churches' importance in society so will come the lessening of their influence.

However, I am afraid there are always people who would prefer the "easy" answer laid down by someone else than the "difficult" answer that actually requires you to do some work thinking. Science will always be the more difficult and demanding route to understanding.