Asking people to reset their device to test out a beta is asking for too much. Most of us aren't glued to our iOS devices 24/7 either, and when we are, it's within apps and media. You're going to have a hard time getting people to see whether adding and removing Notification Widgets works correctly when all they want to do is send messages in Snapchat and browse Facebook. We're not all power users.
The OS X beta makes sense because our Macs aren't just a window to the internet. Most of us interact with our menubar, Notification Center, Launchpad, the Dock, System Preferences, Safari, Mail, Calendar, Preview, etc.
The operating system is important on a computer, whereas the ecosystem matters more on the smartphone and tablet. And the whole point of the iOS beta is to test apps against new APIs, not so much seeing what runs and what doesn't.