I have to say I enjoy ESO's music. I often leave the game on the log in screen just for the music while i work on other projects.

The music is one of ESO's best traits.

when you start bringing lyrics into it, you have a whole bunch of new things to consider. for people like me, having words to process makes it way less dramatic overall -- and i generally think drama is what they're going for in TES music -- because instead of just hearing the themes you're dragging the left-brain into the whole thing just to analyze some vocals, which can take someone way out of a piece. that's apart from the fact that while one person might find some lyrics inspiring, to another it'll sound like a cringey ball of cheese.

then you've got the fact that this game has clients in multiple languages. some people just can't get in on lyrics that don't rhyme, it'll take them right out of the song, and to write words that rhyme in each supported language and are good -- let alone not utter gibberish -- is nearly impossible.

i think that's part of why the Skyrim theme worked: it had vocals, but they were sung in a language that noone processed naturally the first time the game loaded up. the only person who even tried to hear words rather than voices on Day 1 was the cryptolinguicist who had stalked the game designers. most of us didn't even know there was a dragon language for the Skyrim Men's Chorus to be singing in, so the piece flowed like it was just vocals.

i think it sounds like a really hard thing to pull off, and the odds of failure are much greater than those of success, and big businesses don't usually go that route.

I don't think they will ever do this - partly because of what @swippy said above, but also because they change the theme sometimes for major updates - recognisable vocals will limit their ability to do this.