Probably because as described in the linked article IMAP & SMTP is disabled by default - though if you're willing to ignore the scary warnings they're still possible to use.
> Since a couple months ago DDG decided to block TOR Browser users, so I would guess that most people that were concerned about their privacy already moved on to the competition.
I've used the clearnet DDG site for months over Tor and the only problem I've experienced is that sometimes DDG decides that my exit node isn't acceptable for some reason. This is very rare though (happens about once per month) and the error message makes it very clear that I should just use another exit node (unlike many other sites where I get cryptic error messages or a Google captcha).
Plus as others mentioned, there's an onion service which works as well.