Why arm CMSIS library have arm_bitreversal2.asm

Hello,

         I am wondering why CMSIS library have arm_bitreversal2.asm file written in assembly code however it could be written in c as others are written. In description of file it says it written to gain for maximum speed of fft but when I use real fft it calls c function of bit reversal then why it is in c when I do real fft.