So you have to deal with a lot of things. Can the hardware support it, optimized to run on different hardware. Or even if the end user wants ICS. My father for example is perfectly content with gingerbread on his razr. He doesn't want to learn how to use his phone again. Us tech geeks are a small percentage of the smartphone customer base.
You've hit the nail on the head there. Techies and power users want the latest version asap, which is why many on here criticise the prolonged upgrade cycle of Android.
However, for the majority of users its a total non-issue. In my family there are a couple of iphones and half a dozen Android handsets. However, I'm the only techie between us and its only me that regularly changes handsets and try to stay ahead of the upgrade game.
For all the others, they just want a phone that they can use for the regular stuff and will last them for the length of their contract. Do my family give a monkeys about whether they are running iOS 4, iOS5, Froyo, GB or ICS? No.