Plate armor was actually quite capable of stopping a musket ball, especially with the sometimes varying quality of gunpowder, stress of reloading under pressure and range involved and cuirassiers did actually wear what the casual observer might refer to as full plate (As seen in this early 17th century sketch for example). It was acctually not uncommon to test the quality of armor by firing a musket or pistol at it to make sure it held, this is called "proofing".

So why did the practice of wearing plate stop? Muskets are one reason, as muskets became more powerful and increasingly accurate plate armor had to become thicker, heavier and thus more unwieldly to compensate for it.

Another major factor is the changing nature of warfare in the 17th and 18th century. Populations and therefore armies increased damatically in number which led to the rise of more lightly armored and mobile units that were cheaper to equip and train. Knights or fully armored horsemen are a huge investment and of limited use in an age where firepower rose to prominence. Cannons and superior numbers of musketeers behind chevaux de frise along with square formations and other anti-cavalry measures meant that such a small and expensive unit, although still dangerous, would not be economically viable.

Added to this, the number of pitched battles in which a charge of knights to make or break the day was relatively small. Most of the warfare involved a lot of marching around, seeking favourable positions and sieges, all situations where heavy cavalry play a limited role.

The days when cavalry was mostly made up of wealthy people was also coming to an end. Although still the case in the Polish-Lithuanian Commonwealth, France and to some extent Britain, it was no longer the case in Sweden, Prussia, Russia and several other nations.