Part of the reason is, of course, the civil war in El Salvador and geography can play a slight role (although the Bush DOJ believed that so many Brazilians in the early part of the last decade were coming into the US through Mexico that they pressured the Mexican government to require Brazilians to get visas to travel to Mexico and El Salvador is much more densely populated than Brazil), but there is a larger role and that is addressed in this article from 2007:
[Brazilian immigrants] generally come from more urban and educated classes than other major groups of illegal immigrants from Latin America, studies show. Many returning now have been investing their American earnings in Brazilian property.
I don't necessarily buy the language argument as broached by both Weeks and Tyler. Large Brazilian communities exist in many places and not merely where Portuguese immigrants preceded them in locations such as Newark, NJ and various communities in and around Boston. Austin, TX, Mt. Vernon, NY, Danbury, CT and many other locations have significant Brazilian populations. Mt. Vernon in particular has been transformed for the better by the Brazilian community there.
It's been my experience that growth in Brazilian immigrant communities is often endogenous: there are at least 8 Portuguese language masses taking place in the New York metro area, restaurants and stores serving the community grow and develop and influence the larger communities. In my first visit to Red Bull Arena in Harrison, NJ, I was surprised to see such traditonal Brazilian fare as pão de queijo (cheese bread) being sold at the stadium.
When Mércia and I married the plan was from the beginning to some day move (back in her case, to in mine) to Brazil. This has been the norm for some years, especially as I see more and more of our friends returning.