Theory_Man:
One reason,
GW miniatures are made from pewter or plastic.
“Pewter is a metal alloy, traditionally between 85 and 99 percent tin…” - wikipedia
The price of Tin has gone up.
Price of Tin
http://www.lme.co.uk/tin_graphs.asp
There is a good discussion in this thread that relates to this topic
https://discourse.chaos-dwarfs.com/t/5537