This is according to the Kojima Productions Report by Ryan, Report #81. Random anime fan mailed in about it. Reason appears to be that the full space capacity of the Blu-Ray has been used up.
We still have three months to get them to change their minds before they print the discs though! Go and mail both Ryan and Kojima and get them to change their minds!
I’ve sent my mail (also sent this in Japanese to Kojima’s Hidechan):
I heard report #81 and was disappointed to learn that English and Japanese voices cannot be together for the game. Is it possible that you could reconsider this option for the fans? I am sure that the Japanese would also be equally grateful if this option was available.