History

It's not a coincidence that capitalism and colonialism co-developed. They are two parts of the same thing, and have continued to be as colonialism has diversified and been relabelled (imperialism/neocolonialism). Fascism is also part of the continuum of this socio-politico-economic development.

The material developments of proto-capitalism enabled proto-colonialism. Shipbuilding advancements led to Europeans building transportation networks to new places, including the Americas, where it would've otherwise of course been impossible for them to colonize. When Euros first arrived, they didn't really have plans to colonize. As you mention, Columbus the Dumbass was planning on finding passage to Asia, not run into Hispañola. But they were incredibly violent almost immediately, and were already supremely chauvinist. They set to work on "conquering" immediately, beginning with the Spaniards. Columbus returns from his first voyage in 1493. Tenochtitlan falls in 1521. 30 years, basically a single generation, between merely knowing the region exists and defeating its strongest military power. That's not just an outcome of disease or having good technology, it is a decisive and swift adversarial posturing and dedication of resources.

Anyways, getting back on track: as others mentioned, Europe was already primed psychologically and politically for these ventures. Others mention the crusades. The crusades center the church and economic boon through conquering. They have an ideological apparatus that justifies and promotes violence against their enemies along racial and religious lines. The Euros invading the Americas are using that same ideology of casual chauvinist violence and expectations of gains. The system itself sends people to do exactly that sort of thing. And the Spanish famously invest everything in just replicating that process as infinitum, making more ships, trying to conquer more and more places, bringing back more and more gold and resources. Other Euro kingdoms do their best to follow suit, often with the same blessings of the church, beginning a colonial race and contest similar to inter-imperialist conflicts today. Euros constantly fought with each other, kingdoms fell or merged or split, religious factions emerge and wars are fought using them as proxies.

Completing the loop is technology and proto-capitalism. The process of colonizing creates a form of inflation. Gold doesn't have any intrinsic value, you just use it to buy more stuff from other people that expect it to retain value. The church flails trying to regulate this, but the impact is still there. The inflation means you either need to focus on other ways to colonize or you need to get even more gold even faster. Spaniards and a few others try both, and too much of the latter. Others focus on other modes of colonization focused on resources, uneven trade deals, and developing what we now call capitalism. The engine of capitalism drives technological advantage and a feedback loop that makes them militarily dominant, with the ability to make more and more powerful fleets because they have more stuff and better production and more production. This is closer to the style of Anglo settlers as well as the dutch and French, with the former eventually becoming dominant, again with tons of inter-Euro fighting. During the period the Anglos are also colonizing the British Isles as well, for example. It is during this period that the Anglo core industrializes off the deindustrialization of its colonies, a core force developing and being developed by capitalism.

It really is essentially how Marx lays it out, which is as a confluence and dialog between the material base and wider society, with technological development changing economic and social relations, which then respond, and in this case it is the development of capitalism that is the qualitative change produced by the accumulation of these things. And colonialism was an accelerator of this process through inter-colonisr war, technological competition for that war and colonization, and then, eventually, a mode of colonial extraction that favored industrialization of the core at the expense of the periphery. Part of it is historical contingency, like the proximity of the crusades to Euros being able to reach the Americas. Part of it is clear material development, like the fact they could reach the Americas at all requiring technological development and social will. And the two interrelate, as the reason to develop and send those long-range ships emerges from trade questions raised by the crusades.