I think you mean the reverse; equal temperament "just works" regardless of key and doesn't require configuration, whereas JI you have to specify a key, and ther

I think you mean the reverse; equal temperament "just works" regardless of key and doesn't require configuration, whereas JI you have to specify a key, and there are many possible scales depending on which of infinitely many notes you want to include. It's a tradeoff between convenience and optimally consonant harmonies. ET makes a lot of sense for guitar because you have fixed frets (though JI guitars do exist), but violin can be played in any intonation so there's less reason to use ET unless you're accompanying an ET instrument like piano or guitar.