In order, it's probably:
1) Increase in population;
2) Lower barrier to entry thanks to internet/TV (forums like this one!);
3) Increase in disposable income (it seems most guidded sheep hunters are in their 50s and 60s); and,
4) successful F&W management (can't sell tags unless you have something to hunt).
With regards to 1, there may be more people trying it, but there won't necessarily be more people successful (thus lowering success rates overall, but maintaining a fairly steady harvest). It's just easier to go. There are more people who can drop a $1-1500 on travel and a tag than there are people who can drop $7-10k on travel, tag, and a guided hunt.
Although, according to CO, license sales have been pretty steady for the last decade or so:
"Over the past 25 years, CPW has gradually increased the number of licenses sold. Substantial increases in fishing licenses offset the decrease in hunting and combination licenses (Figure 1)."
And as for them not managing their revenue they're doing more today with less revenue: "The revenue we have accrued from licenses sales has increased fairly dramatically, from about $45 million in 1990 to almost $70 million today. However, when adjusted for inflation, the real value of our license revenue has declined over time. $45 million in 1990 would have had the value of over $140 million today. The main reason for this decline is periodic resident price increases that fail to keep up with inflation."
I understand people feeling selfish about their spots, but the state can't focus on the individual hunter anymore than they can manage wildlife for the individual animal. There are too many people who would like to hunt and not enough animals to go around.
Here's a tale of two camps. I belong to a deer camp in the Adirondacks of NY. There are just a handful of camps along the road that is surrounded by a 160,000-acre wilderness area. Our camp was started in 1962 by a group of ex-guides. The two oldest guys in camp joined in 1963 and 1965. The original members are dead or no longer able to get into the mountains. All of the current members joined after the club was started. All hunting is done on foot and on public land. The annual dues are cheap (<$300) and really only go towards maintaining the cabin and paying taxes, propane, etc. The deed and club rules explicitly state that if the camp is no long a hunting club, then ownership reverts to the local township. Right now we're about 20 member strong with a handful of people in every age group from 14 to 73. A few miles down the road is a neighboring camp started around the same time. It's a nice cabin with running water (a big deal) from a nearby waterfall. The original members were very hesitant about taking on new members. It was limited to a few sons and nephews. Eventually life took over and the kids stopped coming to camp. There are only two members still alive. Both are in their 70s and just waiting around for other one to die so that the remaining member can sell the camp.