I think there are two main reasons:

- Early versions of SQL were actually called SEQUEL (Structured English QUEry Language). - Sequel is just much easier to say than SQL, especially within a sentence.

You save a syllable, but also the letters SQL are on the unpleasant side of pronunciation. Just like the word "askew" -- it's just not a nice mouthfeel with that particular sequence of sounds.

But you will insert even more vowels if you pronounce it as "My Ess Kyoo El". Same with HHMMV/Humvee.

History plays a role here. It used to be called SEQUEL before it was renamed to SQL.