Direct mail has a healthy environment. Generally when you're sorting through your postal mail, you're probably a lot more relaxed. This makes a massive difference in receptivity a lot higher than it would be if you were staring at your computer screen, pressed for time, aggressively deleted spam! With direct mail, you're far more likely to get a response because the people reading are happier to read it.
With direct mail, you get a lot more space. This is a benefit because it gives you the chance to tell the whole story. You can include your mailing list arguments for the product, any statistic. More often than not if you write a compelling letter that truly convinces people, they will stay with you. As well as this, you can really emote your feelings to the reader.
You can inspire, frighten, convince, argue, motivate, whatever you need to do, and readers will accept that in direct mail, where they will not in email. In email all that is needed is facts, and this can make for very difficult writing. The only downside with Direct Mail would have to be the cost. It can sometimes be quite costly to distribute what you need on a wide or small scale.