It's completely up to the "buyer in charge of stocking" for the particular truckstop as to what they sell. They do sell the 550 and the 750 in some of them. But because the smaller 450 is cheaper, I notice that many just carry that one.
I noticed them all putting CoPilot Truck 11 on "sale" for $99 too. They are doing that to get rid of existing stock, but the drivers are going to be very disappointed when they find out that version is obsolete.
The big truck stop chains (Pilot, Petro, etc.) often have a "template" which is decided upon by a corporate decision as to what to by.