It's the 21st Century: Outside of Work, Email is dead
To put my contentious hat on:
everyone uses MS Outlook and has done for the last 20 years. It's what the boss tells them to use, so that's what they do
It's the 21st Century: People use Facebook, Twitter, WhatsApp &c. Noone has the slightest interest in archiving years of invitations to go to the pub or links to cat videos. It's not even a youth thing: I'm 58 years old and never email anyone, outside of work.
Independent of whether Thunderbird is good at what it does, it is a product noone needs, addressing a shrinking/dying market.
This is also the reason the UI in Gmail and other webmail is so awful: there's simply no return in spending money making it better.