A few thoughts. First, the 1/2 mile thing is not a "blanket statement." I have successfully worked with Darrel to sanction Spencer Speedway in upstate NY,which is a 1/2 mile. However, this track is about 3 degrees of banking (Flat as can be basically) and it has no walls on the corners. We run a 3.73 in 5th, which, while fast, is not much faster than many 4/10 mile tracks. (Southern National Speedway and Langley Speedway are both 4/10 and use the same gear, and have/will host nationals. In fact, the average speed at Southern National, is almost 5mph higher then at Spencers. (17sec laps at SNS, 23 sec laps at Spencer)
So, it is not 100% that they wont do it. HOWEVER, my thinking, and probably Darrel's too (I am not speaking for INEX, just what I think) Is that 90 mph on a smooth, paved, track, is much safer, than 90 mph on a dirt track where the conditions change weekly, if not hourly. Also, on Dirt you are "hanging it out there" much more, and I believe the potential for severe crashes (Barrell roles, cartwheeling, etc) would be higher.
Just my thoughts. (I am an asphalt guy, so I could be way off)