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Vegas 2 is better attended because it is the 2nd to last race of the season and is an easy sell for the fans traveling in for the race. Many guys back east and up north have wound down their racing season or enough time has passed since their local national event that it becomes viable to attend, the weather is usually good and there is PLENTY to keep the wife busy while he is at the track with the boys. If she comes along, Vegas is the best track amenity-wise (as has been discussed) that you won't have to worry or apologize or explain to the wife about concessions, restrooms, seating, parking etc.
It is not just drag racing ... The locals are apathetic about NASCAR too, people are not coming in for that race anymore like the past either. LVMS is desperate to move that race to the Chase, preferably taking Homestead's date as the season-ender to bundle it with the awards ceremony to pump up attendance for that race too.
EDIT: It will be interesting this year as Vegas 2 is October 24-27, Nevada day is Halloween. We shall see if there is an impact attendance-wise. I say there will be no discernible difference and the place will be packed as per usual. I can tell you for sure there will be 1 more person in attendance ... ME!!!