The boneyards in our SW deserts are home to planes still in use: F/A-18s, 737s, etc. Why did some of these types of aircraft have the unfortunate fate of ending up in the boneyards?

I could only speculate that perhaps the metal fatigue was an issue. Possibly too many T/O and Landings?

Another reason I thought was that particular airlines or the military were scaling back on a type of aircraft, thus having more, say F/A-18s than they need?

I'm curious as to know why.

Thanks in advanced