What about the barrier to entry: $100 + a Mac + an iPhone. As compared to a computer, which pretty much everyone had by 1998. Also iPhone programming is way harder than HTML. I had a mandatory high school class that taught HTML. Maybe I'm underestimating how many people are jumping on this, but my intuition is it can't be as many as were jumping on web. Twitter/social media marketing however... :P
On the dead simple end where you just want essentially a blog with ads, it's easy. On the not-so-simple end, where you want to build an application you can charge money for, the app store model is very straightforward. You don't have to make any decisions about servers or hosts, do any sysadmin work, worry about traffic spikes and scalability, and your cash outlays (aside from development time) are all upfront and totally predictable. Similarly, payment processing is already there and handled for you. And lastly, you don't have to worry about SEO or distribution channels because you don't have a choice in the matter.
It's easier to put up a web page somewhere than to build an app for the app store . . . but for a large number of people building an app for the app store is much simpler than putting up a web application. And that, I think, is a big reason why the app store exploded.