Why do the paisanos in Tortilla Flat speak in the style that they do?
We might presume that the paisanos' speech is actually "translated" from Spanish. Some of the phrases that the men use, such as "thee" and "thy" are direct translations of Spanish phrases that are still in use. (Spanish is more formal than English in some ways.)
This would explain the phrases and idioms used by the paisanos, including the rather formal and codified ways that they curse one another.
The speech patterns of the paisanos serves to emphasize the idea that they do not fully integrate or participate in traditional Western/capitalist culture. They belong to another culture, by choice or by exclusion, or both.