Google wanted Android to be ubiquitous and free to quickly evolve into whatever it needed to be to be in order to capture territory in the mobile device world. That means supporting a wide range of devices, technologies and, most importantly, partners (both hardware, software, carriers, etc.). So Android was designed for flexibility and has extension points everywhere (input frameworks, sharing, alternative browsers, launchers and so on). Google gave up performance along many dimensions in order to maximize Android's adaptability.

Apple is very different. They have a vision and want to build that vision as precisely as they can because that is what has driven their success. That means Apple doesn't want hardware partners. And even at the software level, thsat means they are very careful about who and what they allow into their user experience.

You can see this in how they built new features for iOS 5: Apple added a split keyboard for the iPad rather than adding support for alternative keyboards. They bought Siri rather than exposing relevant extension points. Apple baked Twitter into iOS 5 as a one-off, rather than riffing off of Android's sharing (as they did with notifications). And so on. These are all business decisions, not technical ones (as Cydia often proves). Instead, iOS's technical architecture flows from Apple's goals in the same way Android's flows from Google's.