coffeemug explained it himself in a podcast where he was interviewed about Rethinkdb (I don't remember the name of the podcast). Basically it boiled down to the

coffeemug explained it himself in a podcast where he was interviewed about Rethinkdb (I don't remember the name of the podcast). Basically it boiled down to the fact that all existing databases (at that time) could be tuned with very little effort to take advantage of SSDs so that differentiating factor flew through the window