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.

I only know about Las Cruces from the Organ Mountain Outfitters training material in the DaVinci Resolve sample footage. Sadly they closed a few months ago, which is a shame because I never got my arse in gear to order a shirt from them.