Ah, yes, I had forgotten about that.
Without the approval of the original manga-ka, the work can't be sold overseas. The last volume that was released was probably the last one that had been approved prior to the legal trouble between the author and shogakukan, so now that they're splittsville, Viz is basically SOL.
In fact, this probably goes for worldwide (outside of Japan) rights to the series, plus the license won't be renewed for the existing volumes.
It's sort of sad, but chances are very good that Zatch Bell is dead, permanently, when it comes to non-Japanese publication, because shogakukan owns the rights to the books, but the author would have to work with them to allow overseas sales, and that doesn't seem to be in the carRAB.
There, mystery solved. Unfortunately this isn't really Viz's or anyone's fault, stuff happens, move on.