From what I read in history, they were "trash food", along with oysters. It's funny how so many peasant dishes for low class citizens in the past, have become luxury foods, now served in upscale restaurants.
Many Cajun dishes look similar to Cantonese dishes, despite having different histories and backgrounds. Wide range of ingredients, rice, similar climates, and a heavy utilization of seafood.
Rice is a pretty universal food by and far, like wheat. Africa is another major continent where cultures have been cultivating rice for a long-ass time. And it's been known to Europeans since at least the first century AD, given it had migrated north to Egypt with trade up and down the Nile no doubt. As well, Alexander's men had brought back rice from Pakistan and India on return from his campaigns in Egypt.
The Moors introduced Spain to rice, and from there rice jumped to the Americas where in integrated itself into American dishes (Mexican food is the most famous example). So the use of rice in Latin dishes and Cajun is a side-effect of Spanish influence on the region more than anything.
There's a more complex relationship in regards to then colonial trade in the 17th century. But that's the basic gist of how rice got to the Americas. On a more detailed level rice that got grown in places like the Carolinas came from Madagascar and the old slave plantations of the deep south sometimes grew rice from Africa.
On that related note, I tried my hand at Cajun cooking after I came back from New Orleans. My very white version of Blackened Cat Fish:
It wasn't that difficult to make. All I need to do next time is change one thing to maybe get it closer to how it should be.