In short: Mainly because the actual principles behind aerodynamics were not fully understood kites worked, but why? Later it took some time to develop a powerplant with a suitable power to weight ratio. Gliders are thought to have existed since roughly the early 19th century, but were seen more as a novelty / attraction than a practical means of transport.

I refer you to an earlier post of mine which shortly outlines the development of the heavier than air aircraft:Here

Let me know if this answers your question or that you'd like me to formulate a more in depth answer. That will take some time though!