1. More immigrants from areas where Catholicism was the dominant denomination. 2) Anglicans/Episcopalians were more likely to convert to another denomination or another religion or become non-religious. 3) The colonies attracted British people who disagreed with their government's religion (Anglicanism) and were persecuted because of it. Many had just left Anglicanism or eventually did.

Just observing things, anecdotally, Anglicanism seems to be more popular in Australia than the US even though Australia is a less religious country. So I think it has a lot to do with where people were immigrating from and how they felt about the British government.

Also, I think Roman Catholics are more evangelical. I don't know what their traditions are around proselytizing, but they do some kind of outreach. It's a popular denomination for non-Christians to convert to.

By contrast, Episcopalians are against proselytizing. They specifically tell members to respect other people's beliefs and never directly try to promote conversion. You're encouraged to say positive things about Episcopalianism, and welcome people to come to church / church activities with you, but they really don't want to be like the denominations that are constantly seeking new members. They view that as morally wrong.

Lastly, the Catholic church encourages people to have large families. The Episcopal/Anglican church doesn't. They leave family planning up to the individual. If anything, people are encouraged to have smaller families so they can better provide for each kid.