First, a clarification: North America includes many more sovereign states than just the USA and Canada.
As for why the USA and Canada have been more prosperous than other American sovereign states, territories, etc, there are many factors. My answer will just consider two factors: anti-competitive interventions, and political fragmentation. My answer is grossly incomplete and barely scratches the surface of your vast question, so please just take it as a starting point for further reading.
Anti-competitive interventions
These have their roots in military power, starting with naval power and progressing to more clandestine expressions of power.
From before the Elizabethan period, right through to this century, various south and central American and Caribbean societies have been subjected to powerfully disruptive interventions. These included:
a colonisation perhaps even more brutal than that inflicted upon north Americans (see A Short Account of the Destruction of the Indies (Bartolomé de las Casas, 1552));
looting and other interference with Spanish colonies and shipping by Francis Drake and others;
the support of coups that established dictatorships and undermined democratic civil society in the region;
the imposition of legal and physical barriers to economic interaction.
Political fragmentation
By forming a federal government, the USA was able to reduce political fragmentation within its borders and to create a huge internal market with the same four freedoms of movement as were later adopted in the European Single Market:
- freedom of movement for people
- freedom of movement for goods
- freedom of movement for services
- freedom of movement for capital.
This provided the USA with a reduced amount of internal military conflict (e.g. one civil war, compared to many international and civil wars), and an immense global economic advantage. This economic advantage helped empower the U.S.A. to pursue F.D.R.'s Four Freedoms, which are about as concise a definition of prosperity as one could ask for:
- freedom of speech
- freedom of worship
- freedom from want
- freedom from fear.
Canada's colonisation was less brutal than many colonisations. This, combined with long periods of military and economic protection under the British Empire and, later, in the Commonwealth of Nations, and also in conjunction with Canadian federalism, allowed for both of the above sets of four freedoms to broadly apply to Canadian residents, both within Canada and across the British Empire and, later the Commonwealth.
By comparison, the rest of the American territories have been, historically, more politically and economically fragmented. This means they have had smaller markets available to them, and they have faced greater military threats from other nations. Both these factors reduce the likelihood of achieving prosperity.