Why are 10-pin DIP integrated circuits so uncommon?
While today's computer-assisted manufacturing would make it relatively inexpensive to create and manufacture new styles of lead frames, tooling used to be much more complicated and expensive, and there were relatively few discrete sizes of chips.  As chip production increased, manufacturers may have had to produce new tools simply to keep up with demand (if one has a four punches to produce DIP14 lead frames, one can only produce four such lead frames at a time), but they would have had more inclination to add tools for sizes that were slightly larger than existing ones (so they could offer chips that could do things previously not possible) than for sizes that were smaller.  The only real use for a 10-12 pin DIP would have been for an application where an 8-pin chip would have been insufficient, but a 14-pin part would have been too big.  Even before the invention of surface-mount technology, there weren't many such applications, and once surface-mount technology came on scene there really weren't any: someone who needed a chip with 9-12 useful connections but couldn't afford the size of a 14-pin DIP wouldn't use a 10-12 pin DIP--they'd use a surface-mount part instead.