It was only just before the Wright brothers that people were able to make fairly safe dependable man-carrying kites.
People had been working on lifting people with kites for a very long time. They always started with kites that were far too small, and sometimes advanced to kites that were unstable and far to weak.
Following the development of capable kites, the Wright brothers had some idea about how much surface area they would need, how to make something structurally strong enough to handle the weight and power, and, from rope tension, how much power would be needed.
I’ll also note that landing and take-off are notoriously the most difficult part of any flight. Many early glider experimenters didn’t get much flight time up before they died. Part of the advance was just the accumulation of flight hours, and part of it was accumulated ‘simulator’ hours of various types.
Which leads back to another factor: gliders (and airplanes, and kites) don’t scale linearly. If you make a small model, and it flies well, it won’t work when scaled up. The Wright brothers had the advantage that full-size kites and flying machines preceeded theirs.
Finally, of course the great inovation of the Wright brothers was the new control surfaces. Before that, gliders always crashed.