Dave thinks it’s because Flickr provides an immediate experience: click, the picture loads in almost no time, and then you can appreciate it, comment on it, etc. The podcasting sites don’t offer the same sort of instant gratification. And YouTube? Well, it provided free hosting.
I think Flickr and YourTube have more in common than Dave gives them credit for. YouTube might not provide nearly instantaneous gratification, but it, like Flickr sits at the center of the users attention while they are using it. I think most people consume podcasts like they consume radio. It’s in the background, stealing cognitive cycles from what ever the other thing is they are working on. For that reason, it doesn’t invite the same interaction. This is even more of an issue because some people listen to podcasts on portable devices (like an iPod) that doesn’t allow them to provide feedback.
Update: Removed reference to Dabble.