There are a growing number of email clients (mostly in the mobile market) that ARE supporting HTML5 and CSS3. For the other players in the field, if I had to guess, the greatest hurdle for CSS3 and HTML5 in email is that those things just aren't a priority.

Basically, the email marketer is not really who email clients were built for. The vast, vast majority of people who send email from Gmail or Outlook don't write HTML.

And an email isn't a web page, after all.

I was at The Email Design Conference in Boston last year when a representative from Microsoft Outlook team took the stage and explained that while they were trying to make Outlook more HTML-email friendly, they intentionally use the Microsoft Word rendering engine in Outlook over the Edge/IE web browser engine. This was a conscious decision they had made (years ago) because they wanted office workers who buy their products be able to copy/paste from MS Word and Excel more seamlessly and they prioritized that.

That said, there are tools that can make your life as an email marketer/designer/coder less awful. In our shop we use https://foundation.zurb.com/emails/docs/inky.html which allows you to write more "normal" HTML and it does a pretty good job of converting it into email friendly code. It's not perfect, but that's what testing is for.

I hope this helps a bit.

I question how true this is to today but the argument offered several years ago is that large companies using enterprise software move too slowly and have little interest in the design benefits of HTML5 and CSS3. In other words, large portions of the market are not demanding it.

That should change more rapidly now with the mobile market. Then again there's things like Gmail which still don't care about CSS3. I suspect that's due to a lack of interest.