Because Tumblr, for a large part of its audience, fills the geocities-era niche of self-expression. The majority of tumblr bloggers don't care about 1920-pixel

Because Tumblr, for a large part of its audience, fills the geocities-era niche of self-expression. The majority of tumblr bloggers don't care about 1920-pixel screens, they care about the colors and the aesthetic, web design principles be damned.