One thing that struck me seeing the Queens funeral on TV was that so many of the British dress uniforms belong in the late 19th century or earlier, similar with other commonwealth countries. a quick look at ceremonial soliders in a number of European and South American countries also shows Victorian era Prussian style uniforms, what happened to freeze ceremonial dress uniforms in this era while combat fashion changed so much to suit the needs of a modern army otherwise? Is it just a matter of these being the last 'colourful' uniforms before ww1 made everyone grey and khaki?
More can always be said, but this thread has a series of explanations by u/flotiste regarding the origins of ceremonial and parade uniforms.
I can only speak for the British military, though I believe this would largely hold for the other European armies they interacted with and Commonwealth armies they influenced.
The history of military uniforms is a long battle between smartness and practicality. With each change of dress, there will be complaints that practical changes aren't smart enough - the importance of looking soldierly is paramount, and reflects upon the nation the soldiers represent, and smartness changes are a redundant appeal to vanity, the focus of soldiers should be on soldiering, not fashion.
Through most of history, military technology favoured large formations of massed troops. There would be little point in designing uniforms for concealment. Standardised uniforms, and their precursor in any individual colonel paying to equip their own regiment in matching kit, were primarily designed for opulence and recognisability. Distinct colourful uniforms helped generals observe the field, and made a display of a nation's wealth and martial prowess. Individual units often had quirks in their uniforms to signify events in their history: the Foot Guards adopted bearskin caps similar to those worn by the French Imperial Guard after Waterloo. The Gloucestershire Regiment wore a second cap badge on the rear of their headdress after fighting in ranks back to back in the battle of Alexandria. The Royal Welsh Fusiliers, being the last regiment to hear of the abolition of pigtails (the way long hair was worn at the time), took to wearing black silk ribbons at the back of the neck. All this is to say, uniform was seen as far more than just the clothing of the armed forces.
The full dress worn today has it's roots in the uniforms introduced in 1855. Full dress in the Waterloo to Crimea period is often described as the epitome of fashion over function, and seeing limited changes in this period, were coming to be seen as old fashioned. The experience of colonial conflicts highlighted the need for a more practical uniform, a point reinforced just before the new uniform was introduced (though was under development) when full dress was worn in the Crimean War. As such, the British army dropped the coat, which in one form or another had adorned their soldiers since the army's formation, in favour of the tunic.* The ethos of practicality taking it from double breasted to single after just a year. It is this tunic, though undergoing many small alterations, that has survived as the full dress tunic of today.
As alluded to above, most action seen at the time was colonial, and it was in these conflicts that innovation in uniform was first seen. In hot climates, a white uniform was used. Partly in that white uniforms can be difficult to keep white, particularly in action, but largely due to the changing face of warfare, this was becoming impractical. The proliferation of rifles, and particularly the introduction of breech-loaders, led to an increased importance of skirmishing tactics and extended order formations. All of a sudden, concealment is now a factor. As such, for the course of several campaigns, the white uniform was dyed to a dust colour known as khaki (the urdu word for dust), though at this time it was more of a grey than the greener colour we know today. Originally this was done by the soldiers, though this later was done to clothing before issue, and finally, in 1896, a khaki uniform, now in a hue more (though not quite) recognisable to modern eyes, was approved for all foreign service.
Foreign service, however, is an important distinction. The khaki uniform developed out of colonial service, which had long used undress uniforms and locally acquired supplements. The earliest use of khaki by the British Army was before the Crimean War, and the new, more practical, uniforms appear 20 years after calls for them start to appear in service journals and the like. The colonies appear to be the research and testing ground for the uniforms of the future, whereas conflict between European powers called for the splendour of full dress - each army looking it's best, so as to compete in appearance as well as arms. Over the latter half of the 19th century, this split becomes between home and foreign service uniforms rather than between full dress and undress. Twice in the period, more drab uniforms were trialled on home service**, and on both occasions the uniforms, whilst very practical, were abandoned.
World wars proved inconvenient for the economy, and largely soldiers had but one uniform. The issue of full dress after both wars was retained only for Guards regiments. For the rest of the troops, a smartening up of the one uniform would have to suffice. After the second world war, 'No. 1 Dress' was designed as a more economic full dress, though was not issued on a wide scale. Generally it would be issued only for very special occasions, such as Queen Elizabeth II's coronation. The Second World War saw battledress introduced as a universal, practical (though in many ways this was only in theory) uniform, but was largely considered unsightly, and officers retained the previous uniform to be worn under certain circumstances. It was this uniform that turned into the soldier's 'best' dress - 'No. 2 Dress' - over the latter half of the twentieth century, as it underwent changes and was issued to all ranks.
All this is to say, uniform often stays largely constant, for sentimentality's sake, only begrudgingly changing as practicality demands. A full dress uniform was a way to display tradition as well as national wealth, so when warfare necessitated further departures from it, it's role was retained in a non-combat role, at which point practicality ceases to drive change. The red uniforms we associate with British soldiers today are actually the full dress of only the Brigade of Guards. The best dress of most soldiers, 'No. 2 Dress' is a smartened version of what was worn in the First World War, and similarly, has seen little change since it's introduction as a best dress not worn in combat.
E: Clarification of terms and adding images.
*See the replies below for a further discussion of the difference and the significance of this change.
**I've not been able to find any pictures or illustrations, but the first uniform was issued to the Dorsetshire regiment in 1881, the second in 1884, though I couldn't say who to.
