DTS HD uses the base 5.1 DTS format, and packs the extra data as an extension (more channels, higher sampling rate, etc). The base DTS has to be decoded, and then the HD extensions applied to improve the quality. Where as with Master Audio, the base DTS is there, but it can be ignored and the lossless portion of the data gets decoded.
The original HD extension was messy, but followed in the tradition of all previous DTS extensions (6.1 and 96/24 were both available on DVDs, and would be bitstreamed without any updates to the player needed). So an up-to-date DTS decoder could easily pull the extended fields out and work with them. The Master Audio data is packed differently, and will be ignored by a decoder that isn't specifically aware of it.
By the way, Dolby Tru HD is actually just a tweak of MLP (Meridian Lossless Packing) that was used on DVD-Audio discs (and to send digital data to Meridian speakers before that). So a reference decoder was available before BD players went into production. DTS took a little longer to get their code for DTS HD MA to everyone. Even the first HDMI chips can't bitstream lossless DTS. Luckily the PS3 had enough CPU power available to be able to decode MA into PCM and send that out the older chip to the receiver.
