On Thu, 24 Apr 2008, Roy J. Tellason wrote:
  I've never been clear as to why it was that we
only got 8 slots,  either.
 Though I do have a vague recollection of having seen an ad for some machine
 or other that had something like 12?  Never actually saw one,  though. 
It was based on the Apple ][ physical configuration.
Combined with IBM being too big to ever learn from anybody else's
mistakes.