I had a convo with this about Quesmo(I don't remember his name but you probably know who I'm talking about) and he said it probably comes down to less sales on XB1 means lowering the price might hurt them as the additional sales won't offset the price drop.
However I still think part of it is that MS is a US focused company and deal in USD. For example when setting prices originally, a lot of the time they just deal in USD and convert the rest of the regions. There are some cases where this isn't the case (Witcher 3 for example, but I think you need to specifically request other regions and I think that a lot of publishers/developers just let MS do the converting from USD). I think that because MS deals in USD, publishers just go to them and say "we'd like to lower the RRP to $30" without thinking about other regions (maybe assuming they will convert?) and they do that, but they don't change the other regions to go along with it. Sony on the other hand has departments in EU, JP and USA so they need to get separate prices for all 3 regions.