They wouldn't let Teddy Roosevelt on the ballot in 1912, or Ralph Nader in 2000, so they appear to always have had very restrictive ballot access laws. The only reason Debs did so well there in 1912 was protest votes from people who would have voted for TR.
Somehow John Anderson was able to qualify in 1980, even then he only got 3.3%, as opposed to 6.6% nationally, although he was weak throughout the South.
In 1948 both Wallace and Thurmond weren't on the ballot, but Norman Thomas made it in 1936 while Lemke didn't.
Now as to why the laws are so restrictive, I have no idea.
Debs did no better in OK than would be expected from Socialist strength there during the era: Benson didn't do nearly as well nationally four years later, but did almost as well in Oklahoma. It seems that since TR's candidacy, like Thurmond's later, came out of a post-convention split, that made it harder for them to get their parties on the ballot as opposed to an established party like the Socialists who had their electors all ready to go.
