I really doubt this is the primary reason. I think it was multifaceted, but really the people in the business of designing phones were already using Bluetooth exclusively and felt that it was good enough. Some designs probably rushed it as a means to sell more premium headsets (Apple, Samsung), other saw it as removing an unnecessary redundant feature that was more prone to breakage and a waterproofing difficulty (Google). I think it really just came down to the personal preferences of the (affluent) people who were doing the designing, not necessarily a purposeful cash grab. It really was not great since wired headphones were way cheaper than their Bluetooth counterparts. Luckily Bluetooth has gotten much better and less expensive. I think there should be options for both.