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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"

Perhaps another factor is that transport and communications were so much more limited in the 19th century, that a short tournament would not have justified lengthy travel, particularly for transatlantic professionals visiting Europe. At an amateur level in Britain, the burgeoning train network allowed evening visits from one provincial town to a neighbouring one, so county leagues operated at much the same cadence that they do today.

Today we get tournaments such as:

Chess World Cup 2005 - 128 players
Chess World Cup 2007 - 128 players
Chess World Cup 2009 - 128 players
Chess World Cup 2011 - 128 players
Chess World Cup 2013 - 128 players
Chess World Cup 2015 - 128 players
Chess World Cup 2017 - 128 players
Chess World Cup 2019 - 128 players

Of course these tournaments only last about half the length of time that the one-off 1899 tournament lasted but they occur much more frequently and feature a lot more players, the strongest of which are undoubtedly stronger than the 1899 players.

There are simply far too many very strong players to play double round robins today. Furthermore the players are no longer the leisured rich of bygone days. They have to earn a living either by playing chess, coaching or get a proper job. No organisation or person is going to pay them enough money for 6 or 7 weeks playing in one tournament. We no longer live the Victorian era with its slower pace of life.

One word answer: Money.

1) Tournaments cost a lot of money to organize. At the top end, with, say, less than a dozen players, the organizers are expected to pay for the player's expenses. That's a lot of money. And then a good half of the games being played in the event will be lifeless draws that no one wants to see. The complete score of every interesting game will be available to everyone in the world within hours of the game's end, or earlier. In the 19th century, they could publish a book and gain a little from the sales. Today every magazine in the world will publish their own annotated games from the event as fast as the organizers could, so there's no opportunity to recover any of the money spent on it there.

2) Players want to win efficiently. For example, if they're responsible for their own expenses, they will prefer to play multiple short (weekend) events to a single month-long event, even if the prize money is less, because the net gain is better, and the likelihood of profit more secure.

3) There are far more opportunities to play today than in the 19th century.

Put those three together and it's clear players aren't going to accept invitations to play in 30-round events. They can do better for themselves by playing multiple short events during that time, interspersed with coaching sessions or other money-making activities (Walter Browne used to play backgammon for money, for example) than they will in a month-long event.

Put yourself in their place. Would you want to tie yourself to playing a long event where the chance of a bad start will condemn you to little gain for the entire month, or play short events where you can compensate for a bad game or two in one event with the prize money from the next event?