Leonardo Da Vinci lacked good propulsion. But otherwise people have been toying with replicas of his designs and getting them to fly. Compact, petrol engines did not emerge until late 19th century. And they weren't very light. So, that was indeed a factor. Putting a steam engine on a plane was not going to fly.

Though, they might have invented some kind of functional glider and used a steam powered winch. But, I'm guessing the aero-dynamics weren't that widely understood yet. And making stiff lightweight air-frames also took some time to happen. The contraption that the Wright brothers flew was a bit flimsy. And it barely flew. It was more of a proof of concept. Things like thermals and other updrafts that make gliders work weren't that well studied probably.