In Kalifornia there are three issues that I think are contributing to the decline:
1. The general anti-gun attitude and the difficulty and hassle of buying anything gun-related. Yes, you can still get just about anyuthing you really need, but the gun shops and general sporting goods stores that carry what we need are becoming scarce. There is a 10-day wait for long guns, and you are required to buy a gun lock or prove that you own a gun safe. FFL transfer fees generally run $85 to $125 in the L.A. area. etc., etc., etc.....
2. Rampant development of previously huntable land. The Coachella Valley was/is prime habitat for doves, quail, cottontails, but is now virtually all housing developments and golf courses, with even open desert being annexed by various cities. My favorite close (50 miles) dove/cottontail spot in Chino was annexed by the City of Chino four years ago, but with no signs or public notice. A group I know was dove hunting there and the police SWAT team showed up with full auto weapons and body armor, and had the hunters face down in the dirt while they sorted things out. Not pleasant.
3. Corporate farming has taken over the family operations in the southern half of the state, and that means all the brush (spelled "habitat") gets cleared and all the trees get cut to maximize crop yields, and then they post the ag areas "no hunting." So now we go to Arizona. That adds a couple hundred miles and a couple hundred dollars to each weekend excursion, and that adds up over the course of a season.