Because Reddit has the monolithic "millions of individuals in a millions and ten communities" thing behind it.
Tumblr is a blog site, therefore much more personal and simpler to put a face to.
Basically you're comparing a bunch of, and this is important, anonymous text posts scattered around to someone talking about their personal lives on their blog.
Not to mention you're talking about different internet cultures, Facebook and Tumblr largely miss out on that 25-55 range outside of business and cross-promotion, whereas Reddit largely has the internet generation, architect generation, and first-adopters in it.
So you've got a bunch of people that have no problems with what people say because it's anonymous and often some manner of venting for the perceived wrongs done to them compared to someone that often calls people out by name and face(or username and content as the case may be) for the wrongs perceived done to them.
Also alot of people just use Reddit for its intended use as an aggregate of content and don't bother with comments, again, something Tumblr and Facebook aren't. They're all about comments in comparison.