It's along a feature called the Fall Line.
What happened toward 1570 was an intensification of the generally dry conditions that had prevailed from about 1500 to 1850.
What that meant was that the Tide Water (below the Fall Line) became fairly uniformly salt water. The only freshwater was available above the Fall Line.
Eventually as Virginia was settled the main focus was moved from Jamestown to Williamsburg, and finally to Richmond. Williamsburg was "uphill" and Richmond was above the Fall Line.
It's still the same way. My closest river is the Potomac. It's below the Fall Line. There have been times it was brackish all the way to Great Falls.
The Spanish had their hands full with the Americas. King Philippe II/III opened the East Coast to various groups ~ except the Dutch. It's in the text of the Treaty of London (1604). Otherwise just about anybody in any Hapsburg realm could come here. Once the English figured out what the death rate was from malaria, et al, they did not object to Eastern and Central Europeans coming here.
Well, the Hapsburgs were a bit tetchy about the Dutch, since they had taken away one of their most valuable possessions by achieving independence.