The answer is as easy as it is counter-intuitive: Tumblr is a New York company and Posterous is a Silicon Valley company.
Or, to put it another way: Posterous is an engineered product, while Tumblr is a designed product.
Posterous is extremely well engineered. There’s nothing wrong with it. Every single thing about it is well thought out. But it’s not just that it’s less pretty (though it is). It’s just not designed as well as Tumblr is.
Look at Tumblr’s landing page:
That’s it. Sign up is dead simple. Can you imagine the conversion rates they get out of that page? Moreover, they have one of the best taglines of any service: the easiest way to blog. What else is there to explain? They don’t brag about features like “like”, “reblog” or (ugh) “tumblarity.” Tumblr is the easiest way to blog. Anyone, your mother included, can understand that.
Meanwhile, look at Posterous.
Oh sure, it’s a nice landing page. But, “the dead simple way to post everything”? Sure, it sounds nice, but it’s hard to say what that really is. Is that like Facebook, where you can share all sorts of stuff (videos, links, pictures) with your friends? Is that like Twitter? Or is it like a blog. The “just email us” pun is nice, since all you need to do to sign up is to send an email, but to a distracted user it’s like “What? I have to email them to get an account?”. Then you read all the stuff after that. There’s so much stuff there! A step-by-step explanation, a “who’s it for” (if you have to explain, you’re not doing the right job), a bunch of links and pictures.