Though cast iron was around from the 18th century, they did not truly realise its latent. Ironbridge is a wood composition made of metal and doesn't actually use the features of the element.
But, the key thing was that the Crystal Palace was made of base glass which was not accessible until James Hartley, operating at the Chance Brothers factory in Smethwick, advanced the technology several years before the Great Exhibition of 1851.
Paxton evolved the design for the structure during the early 19th century at Chatsworth on the various greenhouses he built: nobody would have had the idea (or nerve) of doing his ridge and furrow system in the previous century - and his approach to prefabrication and industrialised techniques for the erection of the building were nothing short of revolutionary.
Hope this helped! :)
