Unfortunately the web keeps changing. Firefox is trying hard to remain relevant and needs to be flexible with their code-base. So Thunderbird does not live in a stable world.
Are the people who want it to change confused about the difference between smtp and http?
And as a mail client you'd better follow the security fixes of the web engine you are using.
I guess not everyone is willing to use an old web engine derived from an old version of Word and maintain this thing for the eternity.