It's actually pretty simple: I don't have time. Whereas it takes a split second to evaluate the text, and virtually no effort to skip the parts you're already familiar with - when you're watching the video, you will have to listen to all of the preliminary stuff and bad jokes and stuttering and looking for remote and apologizing for bad English and you name it. Oh, and don't even think of using a video as a quick reference - it's not searchable, either.
Same applies for the discoverability: my ability to find your video is limited by metadata that you provide, and it may not contain keywords for topics that you discuss.
Sadly, this affects even such incredibly educational things as Google's Seattle Conference on Scalability.
And another reason - strange, but fact - video services are often disabled by corporate firewalls.
Bottom line: if you want me to hear you, write it down. Session transcript will do just fine.