One key limit is the speed of telephone line transmission. Videos with a minute or two of sound take an hour or more to download. You'll see quickly that this technology isn't quite ready for prime time. Another limit is the time required to prepare a good presentation. The format benefits the lecturer.
Lectures take relatively little time to prepare, and even less time to update.
Multi-media presentations, web pages, etc. The last multi-media presentation I did took over 20 hours of my time and about 60 to 80 hours of an administrative assistant's time. It was only three hours long. It was impressive. It looked great.