Why do players in the past play much longer tournaments than today's top players? Looking through the history of chess we find tournaments that are significantly longer than today's, e.g.


London 1899 - 15 players, double round robin
New York 1924 - 11 players, double round robin


Today we get tournaments such as:


Tal Memorial 2018 - 10 players, single round robin
Dortmund Sparkassen Chess Meeting 2018 - 8 players, single round robin


For comparison, the players in the London 1899 tournament played a full four times as many games as those in the Dortmund 2018 tournament.

Why such a large difference? Why can't today's players play tournaments as long as those in the past? Alternatively, if they can, why aren't today's tournaments organized to be as long as those in the past?
There is nothing in principle preventing players from long tournaments. Or maybe yes if they are way too long, like the first Karpov - Kasparov match (had to be postponed for health reasons), but these cases are far from the norm.

If we stick to the elite games, I would say that the main reasons why tournaments have become shorter is that there are many more tournaments to choose from! Every organizer wants the best players to play at them, at making the event longer would result in the players occupying a larger fraction of their calendar, so they will either request more prize money or go play somewhere else.

In short, top level chess has professionalized and players will try to maximize their "paycheck"