There are two basic takes on this issue:
1) Physical packaging space and the size of the flash chips -- many people consider this the reason that the iPod touch gets a size configuration before any other device -- it's approximately the same size as the iPhone, and bigger than the other devices, but it has no phone hardware in it, leaving more space for the memory chips. As subsequent memory chips get denser (more storage in less space), then eventually they fit in the more limited space of the iPhone and subsequently the other devices. The nano has a lot less space in it than the iPod touch.
2) Marketing -- that is, Apple pays attention to how many people would buy a larger nano at a price that is attractive to Apple for selling it. So even when the hardware is available, Apple is always playing this game. It's the major reason they're profitable.
There probably will be a 64GB nano. It will probably continue to take 18-24 months for each new generation doubling of the capacity of the nano. There will probably not be a time in the near future when the nano and the touch have the same maximum capacity.
