Producing video courses is costly. From quality lighting, to the beautiful background, to your hair and makeup, there are so many costly moving pieces that must be managed together to make your recordings look even semi-professional. Post-production editing cost is another sore spot that makes many creators procrastinate on even starting to make their course.
In contrast, to produce an audio course, all you need is a microphone and a computer. The less time, money and effort you have to spend on production, the more time you have to focus on the content of your teachings. The substantially shorter turnaround time from concept to product means your teaching gets to market much faster, and both you and your audience benefit.