I have a running theory on why this is the case(2 Theories actually).

The great people who write the software are programmers. To make a great ui you need to be an artist of sorts. And a programmer is no artist in a visual sense(beautiful looking source code aside). That and add to the fact that most of the programs created start out as a command line program. Where most programmers are happy to leave it off at.

The other theory(well not a theory really). Is that there are several major divides in the linux community. For example we have KDE and Gnome(only mentioning major ones here) for the window managers. Both of which operate mostly the same, but operate differently enough that there are PLENTY of heated debates on which is better. The problem i see with that is, we have very talented people on both sides. If only they could combine into one super force, we would be set with one interface that everyone knew(for the most part at least). That would help EVERYTHING IMMENSELY!