Good Question. I think it's because it would be very difficult to name the experiments consistently. If you have characters like "+0-", how would you name the first, second and third level of a categorical variables? What, if there are 4 or 11 categories? How to name a level between + and 0 for a continuous factor? Nobody would be able to understand the nomenclature.
What pattern would you expect to see? If you have an example, you could build a pattern on your own for a more complex than FF and classical designs. But it probably will not work for arbitrary Custom Designs.
See also pattern column description for what is available: Make Table (jmp.com)
Maybe there is a better anwer from JMP staff?
Georg