Times change. The markets change.
Airlines have spent the last 50+ years trying to plot where the market would be in 5 and 10 years.
Each time there is a recession, the market turns quickly and recovers slowly.
Manufacturers merge, get taken over and they phase out models they think will not make enough money to cover the cost of the take over.
Then airlines merge and go bust, leaving surplus frames.
There are so many variables that an upgraded version cannot be guaranteed a market. The 747-800 arrived as twins were proving their worth for long haul. The Hub and Spoke era arrived and has now mostly gone. Regional jets came on the scene. The list of reasons is very long