Hi med1,
1- Imagine that your register is 1 bit long, applying previous question, how many different combinations you may have, 2 or 1?
2- Because the register is 24 bits width.. a different question could be why you cannot set the MSB... and the answer is Nyquist theorem.
3- Is not 15MHz not enough for you? the max frequency is 25MHz...
Regards,
Miguel