Believe it or not, someone asked this very question a few months ago and got a good answer from u/BringlesBeans

https://www.reddit.com/r/AskHistorians/comments/qz2cip/is_there_any_genuine_historical_reason_abe/

Realistically the joke is likely just a throwaway gag about old people having inscrutable beliefs and/or habits (see: onion on belt), but you can certainly make an explanation for why someone would particularly disapprove of Missouri!