One of the big changes was Imperialism - to the point that when Lenin wrote it, if Marx was around he probably would've been like "oh yeah that makes sense" lol
One of the largest differences between capitalism in the 19th century and the 21st century is that it is truly globalized. There are no more frontiers to expand into. Theres imperial super profits to generate, there's maybe some very distant isolated tribes, but there is no where else to disrupt and expand into the way they did during the scramble for Africa or whatever. We are all in capitalist relations with each other now. They still had plenty of feudalism in Europe all the way up to WW1 if you want to pick a date
For why the imperial proletariate in the global metropol doesnt agitate - they do and did but the state is remarkably efficient at destroying agitators. They'll kill anyone who gets close like Fred Hampton. And there's something like a comprador character to the imperial side proletariate, they'll tolerate the international proletariate getting fucked so long as things stay mostly the status quo and they get enough of what we call (derisively) Treats. Between the modern Bread and Circuses and the whip hand of surveillance capital, people dont feel drawn enough together to risk revolution (for now)