For many novice developers, web development imposes its own barriers to entry. First of all, you need some server somewhere, which means you need to know something about servers, maybe have some sysadmin skills, and likely that you'll have to make some sort of ongoing cash outlay for it. Secondly, if you want to get paid you need to figure out how to process payments, or at the very least what ad network you want to attempt to embed.

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.