Because most people don't have network printers? If you've got a print framework, you're developing something which most home users won't be able to use. Your options for connectivity are limited. Developing and integrating it will take developers away from other things.

Perhaps when USB host support gets more mainstream, there'll be print support for USB printers provided in IT OS; but I wouldn't hold my breath. Printer drivers for hundreds of different printers are difficult to write and maintain and take up valuable room on the internal flash.

If printing is necessary for you from your tablet, there are options - but it's not an office tool, so Nokia can't be blamed for not spending money on something which isn't strategic to the tablets' success.