Mastodon is an open platform where people can post things publicly.
Email is private communication between individuals.
Petty disputes exist on Mastodon because admins can see what's being posted and talked about in other instances and so can choose to ban them just for personal spite.
How about your instance admin peeping into everyone's DMs and seeing which servers hold the most people talking against the Democratic party and then blocking all those instances?
Would you like that?
Well, that's what would need to happen with email to achieve a same 1:1 comparison