Imagine you had an icon on your mobile screen that opened just one website, and it was a website already pre-loaded and ready to go on your phone. Just tap it and it magically loaded, using HTML5 with CSS3 and JavaScript. The funny thing about this is that's how Steve Jobs originally intended the iPhone to function, using the concept of Web Apps.
The problem with web apps was that they weren't nearly as fast (and fun) as the apps that came pre-loaded on the iPhone, so hackers managed to tinker with the iPhone to make apps using all the internal goodies of the iPhone. And they started selling them, with Apple getting none of the money, and viruses and malware managed to slip into phones using the hacked-together software.
So Apple decided to allow apps Apple approved to be sold (or given away free) on a newly created AppStore. If there was a dollar amount attached to the sale, Apple got some commission. Apple screened the apps prior to sale to try to keep them safe.
The hackers used the Objective-C that Apple used to make the built-in apps. A lot of web developers didn't know how to program in Objective-C, so the developers had to either stick to HTML5 or learn Objective-C (and buy a Mac, if they didn't yet have one).