Apple products have always been premium devices and as such the brand has a perception of quality.
If looking at iOS Vs Android, then there are a few key differnces;
There are countless manufacturers out there making countless android devices, and these vary greatly in quality and specification.
with a lot of Android based phones youve got the high end high spec stuff that will match and possibly be better than the iPhone, and at the other end of the market youve got the cheap and cheerful stuff that sells for £40 or so.
A lot pf people will have experienced the cheap low end stuff or the mid range stuff that usually cuts corners and feels low/mid range,as opposed to the premium feel/design that you find with Apples range.
As most of the main android device manufacturers have a huge range of phones, and are constantly pushing new models out, in most cases you are lucky to get one update to the OS, as soon as you have it in your hand the next model is being anounced and thats the end of any manufacturer support.
Apple will continue to update older models for substantially longer than most android OEMs
Then theres the manufacturers own take on android, such as touch wizz from samsung, Snese from HTc and so on which can add a further layer of complexity when it comes to updates, from what i understand the Samsung Galaxy S3 will not receive the latest 4.4.x version of android the hardware will run it, but when combined with touchwizz things get a little tight/problematic.