Because the AC3 encoder is more efficient than DTS, like video encoders MPEG4 and MPEG2 encoders, and obtain the same quality with less bitrate.
The AAC encoder is more efficient than AC3/DTS but can't be send by SPDIF to receiver.
I only know this literature about multichannel codecs:
http://tech.ebu.ch/docs/tech/tech3324.pdf