Apple likes to keep their image by controlling their products. The iPhone OS has a reputation for being smooth and fast, whereas other mobile OSs are generally slower - this is due, in part, by the fact that the other OSs can use more than one program at the same time. After running and closing different numbers of applications, over time, the machine slows down. It pretty much comes down to the programmers who designed the code and how efficient and effective their code is. Programmers who developed multi-tasking OSs have a much larger task at hand than a programmer who has the criteria of only being able to run one program at a time.
In other words, if Apple tells you what you can't do, it makes it easier for them to design a product that can do fewer things seemingly (but not always in truth) better.