(The). Profit or money, probably from the game called hazard.
To have an eye to the main chance,
means to keep in view the money to be made out of an enterprise. In the game of “hazard,” the first throw of the dice is called themain, which must be between four and nine, the player then throws his chance, which determines the main.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894