SQL is pronounced 'Sequel' because it was originaly SEQUEL.
SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd[12] in the early 1970s.[13] This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasirelational database management system, System R, which a group at IBM San Jose Research Laboratory had developed during the 1970s.[13]
https://en.wikipedia.org/wiki/SQL
It then later evolved, and changed from being an acronym into an initialism, kind of, sort of, mostly for people who are unaware of the etymology.
'Sequel' is quite literally the tradtional way to pronounce it.