Because, for one, with them came "Pay to Win". Nothing good comes from Pay to Win except that someone lines their pockets.

A professor once told us that something like 1/3rd of people have personalities which are prone to become truly addicted to something. Microtransactions, regardless of their justification[^1], actively target personalities which are especially prone to instant gratification and the endorphins triggered by spontaneous purchases.

[^1]: They _are_ fundamentally justified - it costs money to keep any digital service going, and tons of it for a service like an MMORPG.