This is a little bit like asking why General Patton didn't use quantum phasic shielding on his M4 Shermans, since everybody knew a German 88mm could penetrate American armour. The central problem here is the presumption that, not only was 16th to 19th Century material science aware of the anti-ballistic properties of ceramic plating, but that it was also sufficiently developed to have those anti-ballistic properties and that a figure like Napoleon was significantly versed in this material science to be curious about the development and application of ceramic plating for personal protection.

The first real trial of any kind of ceramic armour came at the end of the First World War, at a time when most belligerent nations were experimenting with personal protection, when it was found by Major Neville Monroe-Hopkin that applying a layer of enamel to a rolled steel plate significantly improved its resistance to small arms fire [Paul Hazell, Advances in Applied Ceramics Vol. 109 (8)]. You'll notice that this armour still required steel plating, which had been largely rejected by most militaries at the time for adding considerable bulk to the already-hefty kit of the average infantryman and significantly impinging on his speed and mobility, as a result being relegated largely to specialist units and static positions. The idea wouldn't really catch on until the Vietnam War, thanks to research pioneered by M. L. Wilkins, C.A. Honodel, and D. Swale [An Approach to the Study of Light Armour, Lawrence Radiation Laboratory, Livermore, UCRL-50284, June (1967)], which again was largely deployed to aircrew in American helicopters in large vests and cockpit plates to protect them from small arms fire during vulnerable moments landing and embarking or disembarking troops in "Hot L.Z.s". Effective ceramic armour requires either advanced modern ceramics, or the combination of ceramic elements with a variety of polymers and lightweight alloys, material science that was several decades away from even being in its infancy during the Napoleonic Wars. The technology to effectively apply an enamel layer to metal - that original idea explorered by Monroe-Hopkin - was only developed in the 1840s, and indeed could only be applied to steel from the early 1900s onwards [Maskall and White, (1986), Vitreous Enamelling: A Guide to Modern Enamelling Practice]. Before this point, therefore, the closest to 'ceramic armour' an infantryman could have got would have been strapping some dinner plates to his chest, which is unlikely to have suggested much by means of offering a useful form of protection.