I imagine that some choose MP2 because the only free AC-3 encoder (within ffmpeg) is not considered to be as high quality as commercial encoders (that pay to use the Dolby libraries). Also AC-3 encoding is a more complex process, requiring things like dialogue normalisation to be set up correctly.

There are some good open-source MP2 encoders (TooLame, MP2Enc), also some video encoders (TMPGEnc, CCE) include MP2 audio encoders.

Also note that some choose to author discs with uncompressed PCM audio, for maximum possible quality.