I don't find Bach difficult to memorize. Perhaps because everything is so... logical! Once you've seen where each voice goes, you can only think like "of course, it couldn't be any different". It's not like some 20th century material which seems kind of random...
It's like a chess position from a real game as compared to a totally random (and even impossible to be reached) placement of the pieces over the 64 squares. Tell an experienced player to look at both positions and then copy them from memory to an empty board. He will surely be able to remember the former (actually it's fairly easy to remember entire games, even those you played years ago), but that "random" position would, with all likelihood, not be perfectly restored.
Bach seems to me so perfect that memorizing is easy. I love Prokofiev, for instance, but to say that it's easier to memorize, say, his sonatas than Bach's fugues is, IMHO, an indicator that the person is trying to memorize the wrong way (note by note, for instance, instead of getting the phrases and their "roles" and vertical relationships throughout the piece), or that one perhaps lacks a deeper harmony understanding. I mean, if you're aware of the harmony motion all the time (like said mikey6), you'll find plenty of 'safety points' everywhere (often every beat is one).
Cheers, all.