Rabbit populations usually run in a 10 year cycles, under normal conditions. However, down here in North Central Texas we have very few rabbits and almost no quail in my area. I believe as do others it is the change in farming practices in regards to fertilizers, pesticide, and herbicide application methods changing.
Now a days they use large wheel vehicles specifically designed to SPRAY (key word) these chemicals on the fields with large BOOMS protruding off either side of the vehicle.
Rabbits and quail alike tend to hunker down, and wait for the vehicle to pass, thereby being COATED with what ever is being sprayed, and this also includes their nests, and their young.
The farmers argue it is the fire ants, but south Texas had fire ants decades before we did, and they have quail and rabbits. To argue this with the farmer, is to lose your hunting rights. They are very defensive, and deny, that now a days they are dependant on chemicals, as they have worn out the land, by seldom rotating crops, or letting it lay out and rebuild occasionally.
Also, I have noticed that ranch land in this area has fair populations of both rabbits and quail, as they do little spraying.