That is a very tough question.
I think it has to do with the data rate you can get with GMSK, which is the modulation used in GSM.
With a bandwidth of 200kz and GMSK modulation, you can get a gross bitrate of 270kb/s. Into how many “slots” can you divide this so that you can still have effective communication? Well, according to the people who designed GSM,the optimum is 8.
270/8 gives 33.75kb/s, and considering that a full rate speech coder requires 13kb/s, you have more than enough overhead to add protection/coding bits.
Hope you’re convinced.
PS: I’m not convinced myself. 😀