The main reason is that you need to be backward compatible with 30 years of terrible hacks. That's why there's a ton of vaporware email clients --- there's a huge amount of work that most of your users don't suspect.

Full-disclosure: I work at a company building an open source email client (https://nylas.com/N1) and an API to simplify email (https://nylas.com/docs)

Maybe because, despite all the noise, e-mail doesn't need fixing? It works, and pretty well too.

I have a project I was at first very excited about but then later paused working on as I discovered more about the email ecosystem. The reason I was excited about it at first was because email is--in theory--decentralized, this part was very attractive to me because I thought this was one potential way to disrupt all existing content silos. However the more I worked on it the more I found out how "centralized" it is when it comes to consumer email. If you take a look around there are only a handful of popular email providers (gmail, yahoo mail, microsoft, etc.) Rest of them are indies. I felt like building a consumer email client (or anything that utilizes consumer email) is as limited as building a twitter client, you're essentially tapping into a big platform. This is why I ran out of steam and put the idea on shelf until I come up with a compelling reason to start working on it again. I wasn't motivated enough.

Sorry, but I personally do not know the answer. I'd like to refer you to dmbaggett of Crash Bandi Coot and ITA Software fame who's currently working on fixing email:

https://news.ycombinator.com/item?id=10732512

This isn't about fixing email, it's about building big things off small ideas.

Microsoft didn't just turn into the biggest BASIC software company.

The point is that the outcome can often be unrecognizable from the origin, and not to worry to much about the endpoint, but focus on doing one thing well at the start.

No-one's "fixed" email because of the network effect. You think trying to break into the social networking space is bad? Consider that everyone who does use social networking has an email address, along with almost everyone who doesn't.