I believe a major factor in this was a ruling by the Hoge Raad that a “free” phone with a contract is unlawful and is actually a loan. Carriers now have to list the price for the phone and for the service separately, so it’s a lot more clear what the costs of the phone are.
Also, a “free” phone is now registered as a loan with monthly recurring costs, which impacts for example the maximum mortgage you can get on a home.