A reason why they went 16 bit instead of 24-bit: efficiency problems with DTS-MA. They had to encode the film twice (2D/3D):
FilmMixer on AVS explains the efficiency differences:
Dolby TrueHD is a complete, "zipped" track of the LPCM that is fed into the encoder. It is a variable bit rate codec.
Quote:
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DTS-HD MA is a different beast. It uses a lossy constant bit rate core. The encoder then encodes the difference between the decoded core and the master, and encodes the difference as the XLL extension. Because of the, shall we say, complex nature of the resulting audio which is being thrown out by the CBR core encoder (i.e. it is mostly low level, but complex, noise) the data rate of the extension may be higher than what the Dolby encoder produces.
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The audio spec thread there shows that the DTS-MA versions are a couple gigabytes bigger, and at least a megabit of bandwidth more.