Any number of reasons I guess - the rookery was out of date, too old? They're late this Spring and will soon arrive. They chose another better place to build nests. They were disturbed by something in the area? New buildings? Trees cut down around them? Noise pollution not usually a problem. Last scenario - shot or killed but this unlikely for a whole flock.
In reply to andrew s:
thanks Andrew there has been building work, but trees are listed so thats less likely. i guess they are entitled to want a change of scenery. it is old at least 50 years and I'm told was the biggest in the area.