A "why not" is rather hard to answer historically, as is often said here. But we can put some things together that may suggest an answer. Before we begin, I will note that I'm going to be working in broad strokes here, which I think I need to do to answer a fairly broad question. There are hardly ever really clean milestones in history, especially on a continental scale, and the use of arms, armor, and uniforms is no exception.

"Proper knightly attire", by which I will assume you mean plate armor, began to decline with the presence of effective firearms. The arquebus became a factor on European battlefields starting in about 1500. While armor briefly became heavier to deal with these early guns, more powerful weapons appearing in the mid-1500s (such as the flintlock musket, which appeared around 1550) started a decline in use of body armor over the next hundred or so years. Breastplate and helmet would survive for foot soldiers through the English Civil War circa 1650, but the British Army was in their familiar red coats by the War of the Spanish Succession in 1700. Light armor would survive in cavalry contexts for longer, the cuirassier only finally disappearing in the beginning of WWI. Full armor even for ceremonial purposes would seem to have fallen out of fashion by around 1750.

Military uniforms, meanwhile, started to show up in Europe around 1600, to pick a nice round number, but were only sporadically used until the New Model Army of the English Civil War. You could say that uniforms became common in Western Europe by 1700. The uniform was one of the attributes of the professional national army that began to become common around this time, and from which our current military systems are descended.

So my thesis on your question would be that full armor was not used at the time that the current military establishments were being created and developing uniforms, and since dress uniforms tend to preserve older styles as service dress changes, there was never a full armor uniform to preserve. You do see uniforms of the cuirassier still present in a few units like the Household Cavalry. You'll note that they trace their origins back to 1660, so their dress would not be any older than that; it actually comes from around 1778, when the unit was reorganized.