The only reason I can think of to answer that question is that his books are a bit different - nicer paper, A5 size, not too long, brilliantly illustrated and good value all round - and so they don't quite fit in with all the others sitting in a bookshop. Perhaps they use up too much space on the shelves.... I dunno.
This is John's version of Beowulf. We bought this along with a couple of other versions (which I will not name) because my son was doing a project about it at school. (Actually we'd have bought it anyway because of who wrote it - we bought the others for comparison.) I read all three and there really is something special about The Geat. It seems to hold together more as a whole story, while the other two just seem to be versions of the original saxon poem and don't seem to be as carefully thought through. For example, only in The Geat does Beowulf get his men to roll around in all the dirt and dung on the floor of the hall so that Grendel doesn't smell them as he approaches - a small point, but an important and believable one. The Geat is also the only version that has any humour in it, even though the descriptions of what Grendel does make it by far the most horrific - an aspect which both my kids appreciated!
I should also mention the illustrations, which, as with John's other books, are by Tom Morgan-Jones. They are beautiful, grizzly, frightening and funny, and with an attention to detail that matches the words perfectly - the pair of them really are a great team!