Expanding a plant is much easier than building a new one. I dont think there is a single community in the country that wants one of these messy, polluting things. Licensing a new plant in 1975 was much easier than today.  
Apart from the EPA and the NIMBYs, expanding old plants is also faster and cheaper than building new ones.  It also allows you to scale capacity gradually up in response to market demands, ensuring maximum return on your investment.  

And there's always uncertainty...  You'd hate to spend 10 years and several billion dollars on a new mega-facility, only to have it come on-stream right when the economy goes into recession and demand is slacking.