I'm going to assume you mean Kansas, I've driven on I-70 and the speed limit is 75. It's also a different state, with a different DOT, different driving conditions etc. Speed limits aren't just pulled out of thin air, they've studied the stretch of road and came to a conclusion 75 is the limits.
There're also a big stretch from KC to Lawrence that runs 4 or 5 wide and when you start getting further out west the traffic isn't as heavy and the landscape isn't nearly as tight and congested as I-80 in Iowa with wall to wall semis.