As people have noted, there are more iOS devices, they are easier/more inviting to use, and have a better browser (more fonts, support for newer standards that Google lags on, smoother scrolling, etc.).
One factor, though, has to be battery life. Android burns through battery—especially if you’re not trained (or don’t have time/desire) to manage your CPU processes. My Android friends can’t believe I have hundreds of apps installed on my iPhone—they’re afraid to install too many because they keep ending up with phones that die rapidly! In fact, they seem afraid of their “beloved” Android phones in general—they lug a charger/cable everywhere, milk their battery carefully, and still run out before the end of the day. Half the time I can’t even reach them because they can’t receive calls.
So they’re hesitant to USE their phones when not absolutely vital. Save that battery for the important stuff! (Who could live that way?) This fear has got to reduce their browsing time.
EDIT: It occurs to me that this factor may be getting worse for Android in the short term: the most-hyped Android phones are those big thick ones using the early-adopter LTE chips! The battery burning 4G chips that Apple won’t touch until better chips come out next year... (Good thing the iPhone 4S can get 4G speed without that 4G battery drain!)