For one approach to answering this, it is worth noting that most of them did collaborate to one degree or another — they were members of the "republic of letters," and traded results, ideas, papers, devices, and so on. For example, if you read Darwin's Origin of Species you can see that his evidence base is largely provided by others, whom he gives credit to, and it has been argued (in Janet Browne's The Power of Place) that Darwin's work would have been basically impossible without the immense speed and efficiency of the British postal service, because it allowed him to mobilize a global network of potential evidence. Even science in Ancient Greece involved interactions with the ideas of previous authors, and that is a form of collaboration.

This is not to say that some of these scientists, like Newton and Darwin, did not labor on individually. They did, even when they were sharing their results with others (and especially when they weren't — both Newton and Darwin had large periods of secrecy in their careers). In general, science became a more directly collaborative effort over the course of its professionalization in the 19th century, when you start to see it concentrated in universities and especially laboratories. (Darwin, it should be noted, was somewhat "old-fashioned" in his approach — he is sometimes called the last "Romantic"-era biologist, who operated more in the mode of someone like Alexander von Humboldt than the biology that was emerging and professionalizing in his later years.)

The versions of this that you see today, where literally hundreds of people can collaborate on a single result (e.g., here's a random paper I picked that has nine pages of author names appended to it) is usually related to what is called Big Science, which a specific mode of scientific research that requires huge collaborations. This more or less came of age in the mid-to-late 20th century, centered around new kinds of scientific equipment like particle accelerators, but looking backwards we can see clear antecedents in the earlier 20th century (e.g. T.H. Morgan's fly lab at Columbia) or even further back (e.g., Tycho Brahe's Uraniborg observatory, which required the labor of many people to make astronomical observations, is frequently referred to as a 16th-century form of Big Science).

Separately we should be very clear that the single-author model often deliberately hides the work of others, especially technicians. Historians Simon Schaffer and Steven Shapin have written at length on the "invisible technicians" of 16th and 17th century science, where some "figurehead" for the whole endeavor (like Robert Boyle) takes credit for all of the work (and is given it in posthumous memory — "Boyle's law," etc.), even though his work would have been impossible without the many people who assisted him with it. Credit issues are huge in science, today and then, and drawing the line for who gets to be recognized as the "genius" behind the work is always controversial. The fact that many scientific honors, notably the Nobel Prizes, restrict the number of potential awardees to a small group (no more than 3 individuals can share a Nobel Prize for a given discovery or invention) creates much angst and occasionally lawsuits, because much of science does not (and has not) operated in that mode.

My last little warning: the popularized versions of science history is dominated by a "heroic" model that is portrayed as being entirely about individuals. This is not how historians of science tell these stories, usually. Beware when you see such models — it is more myth than truth. Even scientists who were relatively solitary still frequently collaborated in different ways with others, often with people whose names most people would not recognize (e.g., almost nobody knows who Marcel Grossmann is, but they know the person who he talked about geometric theories of space with — Albert Einstein).