It’s simply because Country codes in FIFA (and Olympics) aren’t based on English. They use the local lingua franca.
Switzerland’s official languages include French (Suisse), so it’s SUI. If Switzerland were an English-speaking country, it would probably be SWI.
Same reason Spain is ESP (España), not SPA, and Germany is GER (Deutschland in German, but standardized via other conventions). FIFA follows the country’s own naming, not the English one.
Hope you’ve learned something today.