Minimum sample frequency of IMU accelerometer and gyroscope
I'm not sure I can state anything better than Marcus's or Maxtron's comments (it's that way because it's always been that way / because the hardware is limited to that rate), but another way to think about it is:  


The Nyquist rate is 62.5 (=125/2) Hz.  
Can your acceleration or rotational angles (the things measured by the accelerometer and gyroscope) change at a frequency faster than that?


If your instrumented system cannot, then this sampling rate is OK. If your system can move faster, then the sampling will miss something.