There's a lot of moving parts in the equation, but it mostly boils down to Manchu palace intrigue and factionalism on the Chinese side and "Great Game" military sabre-rattling / diplomatic dick waving on the Eight Nations side, who were squabbling amongst themselves almost as much as they were fighting the Chinese.
The Boxers ended up just being pawns of the Dowager Empress's faction of the Qing government, but they started out as a much more subversive and revolutionary movement, like the Taiping Rebellion or the Yellow Turban Rebellion. Every so often in Chinese history when the central government is starting to fall apart, someone will start up a virally popular religious cult based on the same kind of tried-and-true millenialist, end-of-the-world theology that snowballs into a massive, armed and existential threat for the central government.
But it was definitely an "end of an era" kind of moment for all involved. The crushing of the rebellion in Peking led to one of the last great sackings of a major world city in the classical style, complete with immense hauls of priceless treasures which filled the haversacks of even the lowliest enlisted man, and much raping and pillaging all around. And it was one of the last great lopsided colonial campaigns for most of the participating powers, in which they got to match their bolt-action, magazine-fed, smokeless powder rifles and machine guns against smoothbore matchlocks and spears. And for sure it was the last time the great powers were all cooperating on the same side before the whirlwind of the First World War. On the Chinese side, the debacle of the Rebellion weakened much of the institutional influence and political legitimacy of the Manchu imperial government, and the unspoken "mutiny" of the Chinese generals and governors who declared neutrality and kept their highly modernized and Western-trained/equipped armies out of the fighting was basically the beginning of the Warlord Era for modern China.