I would imagine one of the reasons why they have not yet done this worldwide is the contracts they have with internet providers in certain countries. As some people have commented, the cost in Europe is ridiculous, yet the wholesale cost is low and the hotels don't actually make much money from it (according to a GM I know in the UK), it all goes to the provider. So it probably takes time to re-negotiate these contracts / bring in new suppliers as I would imagine that Marriott do not want to pay the £20 a day for platinum members as that is quite a chunk of the overnight rate.