At a certain point, supporting the old install base detracts from your ability to build newer and better product. Everything you design is forced to conform to the lowest-common-denominator standards of the oldest hardware models. Alternatively, you are forced to maintain staffing and resources to continue supporting old devices. This can be a serious roadblock to innovation.

Mobile hardware technology is moving so rapidly that it's almost inevitable to see shorter refresh cycles than you would in a more mature space, i.e., in laptops or desktops.