Presumably because the returns on the investment for making it isn't worth it (either because of low projected sales, cannibalisation of the Pro, cannibalisation of the Air 11", or a combination thereof).
Apple seem to be responding to a general desire for larger screens by inching up the sizes year over year. The basic iPad by next year will have gone 9.7 -> 10.2 -> 10.5 while the Air has grown from 10.5 -> 10.9 and even the mini, the USP of which is its smaller size is supposed to be growing.
Rather than adding a new size model, I think Apple are looking for the largest display size which will still appeal broadly (a lot of people are put off of the 12.9" because they find it unwieldy) and I think they still have room to grow yet before it starts turning people off. The iPad 2 was never considered huge or unwieldy while a 12" display (with original aspect ratio) has almost the exact same footprint as the iPad 2, so certainly I think an 11.5 to 11.8" size would be possible in the future (allowing for a slim bezel).