Because (for the most part) non server-admins don't do backups. It's like flossing; everybody knows you should do it, but nobody actually does it.

For bup, there is something called Kup (http://kde-apps.org/content/show.php/Kup+Backup+System?conte...), but I haven't tested it.

For duplicity, there is a quite good ui in the form of deja dup (see http://www.howtogeek.com/108869/how-to-back-up-ubuntu-the-ea...). It's really nice and easy to use, and if I recall correctly it's installed by default on Ubuntu.

You might want to checkout Arq from Haystack software - http://www.haystacksoftware.com/arq/ - bring your own storage, GUI client, but with open source client in case they disappear, good support, etc.

Being interested in Linux/Unix, file systems, crypto or whatever other technical topic doesn't necessarily mean you are also interested in developing graphical interfaces. That's an art form and discipline in itself. I suppose the people who make these tools are more interested in the tools' respective technical aspects than in interface or graphical design.

Moreover, many of these backup tools run on servers (or as an automated background process) where a graphical interface is more of a handicap than an asset.

I think that in open-source software "pretty interface" sounds often like "customer-oriented" which in turn sounds like "getting paid".

as is the case with most 100% volunteer-based free software: lack of time and/or ppl to help.

There are currently multiple interfaces available to bup, but each one has some quirks. bup's web interface is still very embryonic and would need the magic touch of some designers / integration specialists to make it fun to work with.