I think it basically boils down to the fact that Groundhawgs are old shool technology. The tread compound is really hard, which is good for wear but doesn't grip on hard surfaces very good, they are really loud (all newer mud type tires, even the Swampers, alternate different size shoulder lugs with cancel out a lot of the noise), and don't seem to handle very good on pavement.
Years ago I had a couple of friends running Hawgs and they were down right scary on pavement, especially when it was wet.......though you could do burn-outs all day long with them. One of these guys was into fourwheeling and replaced the Hawgs with the regular bias ply Swampers, he claimed that there was a world of difference both on the road and off (with the Swampers being much better).