Besides the special trash pile stuff with Dark Ages (my next expansion), how does having more expansions make trashing stronger?
Better cards that do things other than money. Trashing is a relatively limited value strategy in the base set because engines have less value, because big money is stronger and engine potential is limited, and because junking attacks are less frequent and less painful. Witch is nothing compared to Mountebank or Ambassador. When you compare the base set's engine potential (village/smithy/market, say) to something like the Golden Deck which is impossible in the base set and impossible without strong use of trashing, or to a Goons engine, or to a deck getting hit by Mountebanks every turn, you see the value of trashing a lot more. Trashing also gets stronger as trash-for-benefit becomes more prevalent - againi, see Bishop, but also Scavenger, Apprentice etc. TFB is very limited in the base set, but can be a dominant strategy on some expansion boards.
When BM is strong, trashing is weak and will tend to seem weak.