Quoted from Tuna_Delight:
The simple fact is that those who have an emotional connection with them (i.e. grew up playing them), and thereby typically appreciate them more, are decreasing in number. The average currently active Pinsider probably can't name more than one collectible wedgehead and most certainly has very little fondness for any classic woodrails either.
Less demand = less value.
I'd consider myself contrary to everything stated here. I'm in my 30's and didn't grow up with 'em. The longer I'm in the hobby, the further back my pinball tastes trend. I also wouldn't consider myself a tech, but I've become far more comfortable working on them than SS tech.
Long live E.M.