The short answer is that email client developers don't have a strong business case to support web standards in their products. Their products are designed and sold as plain text or rich text person-to-person communication tools, not as mass-marketed HTML intrays.
The original web standards movement
The longer answer is that it took a huge effort from some very determined people to get browser makers to support the same technologies and conventions in their web browsers. You can read about the history of the web standards movement here.
The email standards movement
There have been similar efforts to get email client makers to adopt web standards, notably the Email Standards Project started by Campaign Monitor. They developed the email ACID test to test email client support of HTML and CSS features, and they publish a list of email clients including support levels.
Email client developer reactions
The Email Standards Project were very active for a time, launching fun campaigns like Fix Outlook and the Gmail Grimace, and even getting the attention of the Outlook team at Microsoft, who hung a "fix Outlook" poster on their office wall.
What's the business case for web standards support?
For email client developers, there does not appear to be an obvious business case to support web standards. Their applications are generally used for rich text emails; advanced HTML support isn't essential for day-to-day business communication, even though marketing companies and web designers would like to think it is. Web-based email clients such as Gmail and Hotmail, and aging clients such as Lotus Notes 8 and Outlook 2007 are unlikely to improve in the immediate future.
It's still worth putting pressure on email client developers, though, and you can help them spread the word here.