From what I understand, sixteenth and seventeenth-century European and colonial American soldiers generally wore cloth or leather hats, often tricorne or bicorne, but in the early-nineteenth century they switched to wearing shakos. What prompted this change? Was it practical or cultural in nature?
I've written about this previously here. In short it was because felt tricorne hats were impractical, offering no protection from the weather. The light infantry troops that became more common at the end of the 18th century also found that they got in the way. Leather helmets were tried but they proved expensive and uncomfortable. Shakos had the advantage of being cheap and practical, and became fashionable at a time when massive armies were being raised thereby becoming ubiquitous.