Cotton seed oil was cheaper, look it up. While cost was certainly a factor, scorch point was also a big player in just how dark a roux you could get without burning. Lard will burn at lower temp, btw. Used to be quite a bit of cotton back in the day and the seeds that made the oil were a worthless byproduct.
When it comes to potato salad AND okra in a gumbo, I'm Bi-sexual. I can eat it with or without. EVERY chance I get. Bend ova mfer.