It is because far too many people fail to account for climate change. The overuse of analog years also contributes to poor forecasting. Using analog years would make some sense if the climate had remained essentially the same.
The source region for the cold air has warmed considerably.
Larry Cosgrove is one of the few Mets who mentions that climate change has to be a consideration when developing a seasonal forecast.
Another thing that leads to poor forecasting is a lack of pattern recognition. There is an over reliance on the models and various indexes such aa the AO PNA and NAO.
Just look at what has been occuring....one system after another into the Pacific NW. Back in the day forecasters on the east coast would often mention that a stormy pattern for the Pacific NW doesn't bode well for winter weather enthusiasts on the east coast.....now it is rarely mentioned.