Strange that in “preview” mode it copies you all the audio tracks of your original MKV, while converting the video leaves you only the first track!
However, I can answer you in part:
because simply the program still needs to be optimized for handling video files with multiple audio tracks present at the same time on the same video file… in fact it is inconceivable to me that the possibility to choose at least the audio track to be converted has not yet been added to the program GUI.
In fact, this is precisely why I created an official request to add this feature:
