I read that the modern kepi arose from the styles of the shako, due to the shako being disliked by french troops during the Franco-Prussian wars. Why would they dislike their shako hats?
The soft kepi was light and comfortable, whereas the shako was heavier and was uncomfortable to wear. The troops had kepis with them as undress headgear, so tended to wear them instead, rather like the Austrians in 1859 tending to wear their undress lightweight white kittels rather than their dress tunics.