The main reason was extremely low light sensitivity in the early photographic materials. Early plates such as daguerreotypes were vastly less sensitive to light than later film or modern digital sensors, so they needed much more time to record an image.

One community answer compares a daguerreotype to roughly ISO 0.001, which is extraordinarily slow. By comparison, even older consumer film was much faster: common color films in the 1960s–70s were around ISO 25, and ISO 100 was considered fairly fast for color film.

With such low sensitivity, early cameras had to leave the plate exposed for minutes or longer, especially in anything less than very bright sunlight. That’s also why moving subjects often disappeared or blurred away, while buildings and other stationary objects remained visible.

So the short answer is: early photos needed long exposures because the chemistry of the recording medium was extremely insensitive to light.