Currently, the only audio choice for applehttp transcoding is aac. While aac support multi-channel audio, due to licensing issues all Roku media streamers convert multi-channel aac to two channel PCM. Since applehttp is the preferred method of transcoding for Roku players (allows fast forward and rewind capability), it is desirable to be able to retain multi-channel audio whenever possible.
Remember that it is possible that the audio codec being transcoded to have more than 5.1 audio. It is common for Dolby TrueHD or DTS Master Audio to contain 7.1 channels, and with Dolby Atmos/DTS:X encoding even more channels could exist. So support for a channel count higher than 5.1 is necessary.