The restriction goes to 18k because that's the top of VFR space. Anyone operating above 18k has to be on an IFR clearance and under positive ATC control. That makes it easy for the feds to make a call and say "Hey, center, get everyone out of this airspace" wheras in the VFR altitudes it's very difficult for them to legally clear the space since a VFR plane could be flying around not talking to anyone.