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?