They tanked ultra-high-value items, which were primarily traded off-the-books because their value exceeded the official Steam marketplaces $1800 price cap. Bringing those prices down is good for Valve because it means more trading activity will happen on the official market, where Valve gets a cut of every transaction, rather than on third-party exchanges, where Valve gets nothing.

Simply raising the $1800 transaction limit and $2000 balance limit would have been far less disruptive, but that may have put Valve in financial regulators crosshairs. There's surely a reason why they chose those numbers in particular.