Why do Flight Replicas aircraft stipulate the use of Acceleration, and not just SP2?

SP2 and Acceleration were developed by different teams within Microsoft. SP2 was a temporary fix to earlier problems within FSX, and Acceleration did the same fix but also supplied many more features to FSX and changed certain aircraft parameters, and in our opinion made it a more accurate and much better experience. As such, the code is different in each, which is why our newer aircraft aren't always suited to running with the older SP2.