The article states that they initially tried to do this with mail forwarding contracts, but those really aren't designed for this kind of use case (more on this in another one of my comments[1]). To accept mail on behalf of someone else requires a much more complex signup process: you have to get a special form from the postoffice signed in the presence of a notary public and then send that in physical copies to various parties; to get mail forwarded is much simpler. Earth Class Mail tries to help you with this, but as a happy user I honestly had kept putting off signing up due to it: I imagine these services could easily have many more customers if they didn't need such a "hard" form.
I imagine, then, that it primarily comes down to this attempt to minimize the perceived pain of the signup process: specifically, the idea that you have to move all of your mail senders to a new address is really off-putting to potential customers; it is effectively like moving from your current home "to" the other provider. I was myself a little against doing this at first, and in fact only came around when I finally physically moved to a new apartment and realized "omg, this is so annoying, and I am not even certain I like my new place, so maybe I'll move again in a few years... I'm going to do this once to ECM instead and maybe then I won't have to do this for another decade".
At that point you first look at mail forwarding, and then fall back to "undeliver" while you attempt to argue people into submission that mail forwarding should let you do what you want, or try to get the government to pay you to replace their existing service with yours ;P. But to try to say "our service is so easy: all you do is contact anyone who ever sends you mail, and have them all send mail to us instead! then, if you ever get tired of paying us to receive your mail, you get to do this all over again!" is a really difficult sell, especially for a "hip" "web 2.0" company. I almost want to say they had no other option but the path they took (and no other option than the resulting failure).
[1] https://news.ycombinator.com/item?id=7668877
I imagine, then, that it primarily comes down to this attempt to minimize the perceived pain of the signup process: specifically, the idea that you have to move all of your mail senders to a new address is really off-putting to potential customers; it is effectively like moving from your current home "to" the other provider. I was myself a little against doing this at first, and in fact only came around when I finally physically moved to a new apartment and realized "omg, this is so annoying, and I am not even certain I like my new place, so maybe I'll move again in a few years... I'm going to do this once to ECM instead and maybe then I won't have to do this for another decade".
At that point you first look at mail forwarding, and then fall back to "undeliver" while you attempt to argue people into submission that mail forwarding should let you do what you want, or try to get the government to pay you to replace their existing service with yours ;P. But to try to say "our service is so easy: all you do is contact anyone who ever sends you mail, and have them all send mail to us instead! then, if you ever get tired of paying us to receive your mail, you get to do this all over again!" is a really difficult sell, especially for a "hip" "web 2.0" company. I almost want to say they had no other option but the path they took (and no other option than the resulting failure).