Well, I dunno if this has anything to do with it (I lean toward the more-money-and-time-theories above, actually), but...
A lecture is an anachronistic way of sharing information today, in my opinion. Like most people, I can read faster than someone can talk, so it's slow, compared to the written word. Also, it requires an investment in travel to get everyone gathered in the same place at the same time, making the dissemination of information very inefficient. That can be solved by Youtube, of course, but there's no way to look out over the "audience" and judge age there.
Also, a lecture is usually not interactive, in any efficient way. If an audience member disagrees with an assertion in the lecture he/she might be able to ask a question afterward. But the lecturer usually can only reply with a short response and no documentation because he wasn't prepared for the question, and the questioner can provide no documentation either because she didn't know the specific point would come up. Compare that to a slightly slower but far more complete electronic conversation, full of links and images of primary sources and more detailed analysis.
So the lecture format is, primarily, an "expert" slowly conveying information to a passive audience who have spent a lot of effort to hear him, and who aren't expected to question his wisdom. Not my cup of tea. I want speed, efficiency, interaction.
Now, here's the problem. I've always been that way, even before the computer age. The computer age fit exactly with what I wanted. I'm now one of those old dudes (55) who should be in the lecture audience and talking about why lectures are fun, but instead I'm arguing why I don't like them.