Multiple people have said it is a ONE-TIME link. Meaning only ONE PERSON can download it. It is a space issue, they'd need to have a big server with lots of space to hold all the files. The systems themselves cost a lot of money, on top of that, they'd need to buy all the hard drives for the servers, those cost money too. Then they have to supply power to all the servers, and have a fast enough network. All of this costs money. Also, someone has to put in the time to make sure that it automatically gives you a download link for your world. Given all of this, I don't think it's worth the time or money to have every single world from the past 5 years saved forever and ever.
You said your friend made a world download, ask him for the world. If he doesn't have it, ask your other friends. If they don't have it, then that's it. There's nothing you can do.