I wonder how many of his original blog's visitors were just bots. Also social networks have really lowered the barrier to entry for posting your thoughts online. The marketplace of ideas is much more crowded.
While the marketplace is more crowded, the number of markets has shrink. To continue your analogy, each marketplace vender used to be its own market, so you had a lot of options. Now there were really only 2-4 giant marketplaces, and vendors with a good location have made it really difficult to set up shop.