My guess is because their OS is programmed for that one piece of hardware and so it's streamlined to be as efficient as possible. Android is a nasty memory hog and memory manager. To get Android to run smooth you need to throw a lot of RAM at it. Think of it as the difference between a game console and a PC. The 360 had 512mb RAM and a comparatively crappy video card. Put that much RAM and the equivalent card into a PC and you couldn't run with anywhere near the graphics performance of a 360.
It's all about closed architecture and being more efficient. I'll still buy Android devices all day erry day though.
^That. Most of Apple's hardware is designed and made specifically to work in conjunction with their OS, and because they are the only ones able to create phones using that OS, there is no variation. Android devices, on the other hand, come in many varieties, from as many manufacturers, and each one decided to put their own little spin on the OS. Its hardware components end up looking like a Frankenstein monster next to Apple's. That all adds up to it being not nearly as efficient or uniform. It's a small price to pay for a little variety though.
The main reason is android phones are made by different manufacturers whereas Apple is the only one that makes iphone. This means that they have full control over the os. Also ios is a lot simpler in the way it runs. Android phones on the other hand have different skins which will make the hardware perform differently. By putting a lot of ram. they are safeguarding it so that it can run as long as possible even with future features.