Very little configuration required seems to be one main idea. The most important reason I have been interested in Tox is that I got the impression early on that they intend to make it resistant to traffic analysis as well (and judging from this thread http://4chanarchive.net/threads/g/Tox-Thread/43340618 there are still such plans) but that isn't emphasized in the page at tox.im any more.

I'm guessing they're prioritizing getting something that the average Skype user would be capable of installing and configuring, and saving the onion routing feature for later.