So, I have a slight feeling you may be conflating the ideas here.
Evertune keeps the string in tune with itself, despite what you do while playing - extreme bends, lots of whammy etc. won't send the string way out of tune. If it was an E when it started, it will still be an E.
True temperament is that the whole fretboard will be intonated near-perfectly for the temperament that it is made for.
The answer to your second question is fairly straight forward, and also fairly complex. For the most part, it boils down to cost. For, say, Fender, to adopt either system, would require re-tooling their production. Cutting straight fret slots is very simple to do. Cutting the "curvy" true temperament ones is not, and I also believe they'd have to license it as well, adding more cost.
Also, I believe that the overwhelming majority of musicians don't see the need or point of going with true temperament. The drift in intonation with standard necks that have been in use is perfectly acceptable.