I've noticed that the seat maps tend to be very poor indicators of real availability on the LAS flights. I've seen plenty of instances when the flight was completely sold out in F but there were 3-4 unassigned seats. This could be the casinos holding seats for high rollers (though not sure how they do it without a traveler's name).

Also, keep in mind that LAS is a big convention city, so there are a lot of business travelers too, and many of them are allowed to purchase F seats (or Y and instant elite upgrade).
As has been discussed elsewhere in the forum, CO often doesn't release upgrade or award seats until closer to flight time when it becomes less likely that they will sell the seats. Automatically assigning 2 upgrade/reward seats to every flight 330 days out doesn't seem like a particularly smart business decision in terms of cash flow.

You could also try purchasing Y and getting an instant elite upgrade, it's only ~$1100 on that route, not too bad (or full F is only $100 more than that and you get the 50% COS bonus miles).