Probably because nobody would agree the format! I like the HSE's take on a matrix ie a simple 3x3, (low medium, high). Others where I work believe that 4x4 or 6x6 means that people don't plump for the middle options. I've even come across organisations that get something (not sure what!) out of a 10x10 matrix.

Ian, If you use a 10x10 matrix you it would give you a risk as a % giving a mathematical thinking layman some idea of what risk is involved. I have used it before and worked very well and does seem easier to understand for some. What you should remember though, it is the risk controls that are important and not to get too hung up in a matrix figures.