Because for the average consumer, 2 retina full sized iPads which would be priced different would surely confuse people, and why buy a 4th gen with the minimal increase of speed and new port, when you could get an, to the average consumer, identical, except port, iPad 3rd gen cheaper?
Instead, the iPad 3rd gen was discontinued, and the iPad 2 kept around as a budget iPad, where upgrading to retina would seem appealing.
Also, their flagship iPad needed the new port, so they jam packed a little something something in there too just to make it maybe seem worth the upgrade from 3rd gen.