The article argues that it's largely a result of the demographics. Tumblr is primarily a very young audience with, "The percentage of 18-to-24-year-olds on Tumblr is roughly double the internet average." And that no doubt has a lot to do with it. Young people on average are far more likely to support Obama than Mitt Romney. However it's not just about the people who are on tumblr, just looking at the debates on abortion shows that there is definitely a constituency of people who have similar ideas to Mitt Romney, why aren't they taking part in Mitt Romney's Tumblr campaign?
Tumblr users pride themselves on being part of a community and in order to be a part of one there has to conversation and that means a give and take. Obama's campaign got it right because they interact with the community, reblogging and replying to their posts. So while they still use it as a platform to distribute their message, they also use it as a playful way to keep in contact with their supporters.
See this post where someone posted a picture of an Obama LGBT sticker on a wall saying "I've been meaning to get one of these for my wall" and the Obama Tumblr responding by saying "And now we, in turn, covet a cupcake wall sconce for the office." with the hashtag "seriously where did you get that amazing cupcake wall sconce?" It currently has over 800 reblogs and likes.
Being able to adapt to each platform online is an important part of social networking and in this race, Obama is clearly winning.