it has not evolved much. and i believe that's a good thing. this is the reason why emails from 20 years ago look the same even if you open them today. given the short life of everything on the internet, email is a wonderful thing.
Yes, I have no idea why UI practices need to constantly "evolve". As far as I am concerned a lot of UI is getting worse over time so the less evolution the better.
There is some need to evolve as contexts change. Screen sizes and capabilities, computing speed, available bandwitdth etc.
Also humans seem to need fashion as a mechanism to show belonging and distinction. (Differentiate form parents generation, show affiliation to some sub culture, ...)
And then there is the hope to make the production more efficient. (Which may not work out)
There are periodic items on here from people having trouble with self-hosted email, but nonetheless I think it's also not suffered the same closed-systems/walled-garden effect as much as other protocols like IM.
