Because you use "An" if the following starts with a vowel sound, and "SPA" (spoken as the individual letters) does.
(EDIT: fell for a false friend, fixed)
Sounds similar to the battles between es-queue-el and sequel for SQL.
https://abbreviations.yourdictionary.com/reference/abbreviat...
https://www.merriam-webster.com/words-at-play/whats-an-acron...