I don;t think you really fully understand the relationship between 4chan and tumblr, specifically with /co/. it is a misconception that has been perpetuated among newfags since 2011, and the cancer therein has become more and more concentrated, much like a man drinking his own piss.
Before 2010-2011, tumblr was not known for the SJW craze that currently infects it. It was known back then as aboard for hipsters, where everyone who wanted to feel special about their obscure tastes flocked to (sound familiar?). in the early days from late 2008 to early 2010, that was its primary repute. it was also home to many artists (and still is), as most late teenage/young adult artists did not want to stagnate around the prepubescent communities of DevieantART or Gaia. This separation from juvenile weeabooism estranged these people form anime, and as a result, most of their art was western-centric. Naturally, /co/ became intimately linked into this, and tumblr became our primary source of art and drawfags. In short, is wasn't tumblr that took over /co/, it was /co/ that took over tumblr. This set the stage for further developments over the next two years.
Come the 2012 election cycle, and political temperatures were running high. people who mere moderate stated to become more and more militant (This would become quite important to know four years later). As we all know, the vast majority of hipsters are liberal. Modern liberalism goes perfectly with the self-aggrandizing, self congratulatory attitude of hipsters. When the 2012 elections reared their ugly head, so did their inherent leftism. they evolved, and became the SJWs we know and hate today. In the meantime, /co/ was trying to distance itself from the whole mess, all while dealing with the brunt of the blame for the horse incident, which it indeed started, but didn't propagate.