At a certain point, materials science hits a brick wall until new technological developments are made. The growing power of muskets and pistols slowly made it harder and harder for armor and shields to reliably deflect bullets while also being light enough to reasonably wear/carry. While cuirassiers would continue to wear armor on their chest (hence their name), the weight of steel armor that could protect against a musket quickly became untenable, especially to issue to everyone. Lighter forms of armor like mail and leather would of course be useless against direct hits from guns, so there's no need to bother issuing it even if you can.

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Body armor only re-entered the world en masse in the mid-20th century with new materials that prior generations never could have discovered, like Kevlar. Harder steel was developed by modern metallurgy, allowing for AR500 steel plates that can absorb powerful rifle rounds. Ceramic was found to absorb energy well and now we also have ceramic plates. Titanium wasn't even economically feasible to produce in the amounts needed for armor until recently.